lang: Add module imports and more

This enables imports in mcl code, and is one of last remaining blockers
to using mgmt. Now we can start writing standalone modules, and adding
standard library functions as needed. There's still lots to do, but this
was a big missing piece. It was much harder to get right than I had
expected, but I think it's solid!

This unfortunately large commit is the result of some wild hacking I've
been doing for the past little while. It's the result of a rebase that
broke many "wip" commits that tracked my private progress, into
something that's not gratuitously messy for our git logs. Since this was
a learning and discovery process for me, I've "erased" the confusing git
history that wouldn't have helped. I'm happy to discuss the dead-ends,
and a small portion of that code was even left in for possible future
use.

This patch includes:

* A change to the cli interface:
You now specify the front-end explicitly, instead of leaving it up to
the front-end to decide when to "activate". For example, instead of:

mgmt run --lang code.mcl

we now do:

mgmt run lang --lang code.mcl

We might rename the --lang flag in the future to avoid the awkward word
repetition. Suggestions welcome, but I'm considering "input". One
side-effect of this change, is that flags which are "engine" specific
now must be specified with "run" before the front-end name. Eg:

mgmt run --tmp-prefix lang --lang code.mcl

instead of putting --tmp-prefix at the end. We also changed the GAPI
slightly, but I've patched all code that used it. This also makes things
consistent with the "deploy" command.

* The deploys are more robust and let you deploy after a run
This has been vastly improved and let's mgmt really run as a smart
engine that can handle different workloads. If you don't want to deploy
when you've started with `run` or if one comes in, you can use the
--no-watch-deploy option to block new deploys.

* The import statement exists and works!
We now have a working `import` statement. Read the docs, and try it out.
I think it's quite elegant how it fits in with `SetScope`. Have a look.
As a result, we now have some built-in functions available in modules.
This also adds the metadata.yaml entry-point for all modules. Have a
look at the examples or the tests. The bulk of the patch is to support
this.

* Improved lang input parsing code:
I re-wrote the parsing that determined what ran when we passed different
things to --lang. Deciding between running an mcl file or raw code is
now handled in a more intelligent, and re-usable way. See the inputs.go
file if you want to have a look. One casualty is that you can't stream
code from stdin *directly* to the front-end, it's encapsulated into a
deploy first. You can still use stdin though! I doubt anyone will notice
this change.

* The scope was extended to include functions and classes:
Go forth and import lovely code. All these exist in scopes now, and can
be re-used!

* Function calls actually use the scope now. Glad I got this sorted out.

* There is import cycle detection for modules!
Yes, this is another dag. I think that's #4. I guess they're useful.

* A ton of tests and new test infra was added!
This should make it much easier to add new tests that run mcl code. Have
a look at TestAstFunc1 to see how to add more of these.

As usual, I'll try to keep these commits smaller in the future!

etcd/deploy.go

@@ -91,32 +91,53 @@ func GetDeploys(obj Client) (map[uint64]string, error) {
 	return result, nil
 }
 
-// GetDeploy gets the latest deploy if id == 0, otherwise it returns the deploy
-// with the specified id if it exists.
+// calculateMax is a helper function.
+func calculateMax(deploys map[uint64]string) uint64 {
+	var max uint64
+	for i := range deploys {
+		if i > max {
+			max = i
+		}
+	}
+	return max
+}
+
+// GetDeploy returns the deploy with the specified id if it exists. If you input
+// an id of 0, you'll get back an empty deploy without error. This is useful so
+// that you can pass through this function easily.
 // FIXME: implement this more efficiently so that it doesn't have to download *all* the old deploys from etcd!
 func GetDeploy(obj Client, id uint64) (string, error) {
 	result, err := GetDeploys(obj)
 	if err != nil {
 		return "", err
 	}
-	if id != 0 {
-		str, exists := result[id]
-		if !exists {
-			return "", fmt.Errorf("can't find id `%d`", id)
-		}
-		return str, nil
-	}
-	// find the latest id
-	var max uint64
-	for i := range result {
-		if i > max {
-			max = i
-		}
-	}
-	if max == 0 {
+
+	// don't optimize this test to the top, because it's better to catch an
+	// etcd failure early if we can, rather than fail later when we deploy!
+	if id == 0 {
 		return "", nil // no results yet
 	}
-	return result[max], nil
+
+	str, exists := result[id]
+	if !exists {
+		return "", fmt.Errorf("can't find id `%d`", id)
+	}
+	return str, nil
+}
+
+// GetMaxDeployID returns the maximum deploy id. If none are found, this returns
+// zero. You must increment the returned value by one when you add a deploy. If
+// two or more clients race for this deploy id, then the loser is not committed,
+// and must repeat this GetMaxDeployID process until it succeeds with a commit!
+func GetMaxDeployID(obj Client) (uint64, error) {
+	// TODO: this was all implemented super inefficiently, fix up for perf!
+	deploys, err := GetDeploys(obj) // get previous deploys
+	if err != nil {
+		return 0, errwrap.Wrapf(err, "error getting previous deploys")
+	}
+	// find the latest id
+	max := calculateMax(deploys)
+	return max, nil // found! (or zero)
 }
 
 // AddDeploy adds a new deploy. It takes an id and ensures it's sequential. If