lib, lang, docs: Add --only-unify option

This adds a new run flag for the lang frontend to exit immediately
following type unification. This makes it easier to use this as a step
in CI, and also to type the execution for performance comparison
reasons.

docs/faq.md

@@ -216,6 +216,21 @@ requires a number of seconds as an argument.
 ./mgmt run lang examples/lang/hello0.mcl --converged-timeout=5
 ```
 
+### Can I run `mgmt` for type-checking only?
+
+Yes, you can, add the `--only-unify` option to the lang frontend while using the
+`run` command, and it will exit after type unification.
+
+#### Example:
+
+```
+./mgmt run --tmp-prefix lang --only-unify examples/lang/hello0.mcl
+```
+
+It will also print how long it took on either success or failure. Keep in mind
+that even if you pass type unification, an `mgmt` run can still fail later on
+for other reasons, although these are mostly runtime considerations.
+
 ### Why does my file resource error with `no such file or directory`?
 
 If you create a file resource and only specify the content like this:

lang/gapi/gapi.go

@@ -24,6 +24,7 @@ import (
 	"fmt"
 	"strings"
 	"sync"
+	"time"
 
 	"github.com/purpleidea/mgmt/gapi"
 	"github.com/purpleidea/mgmt/lang"
@@ -97,6 +98,10 @@ func (obj *GAPI) CliFlags(command string) []cli.Flag {
 				Name:  "update",
 				Usage: "update all dependencies to the latest versions",
 			},
+			&cli.BoolFlag{
+				Name:  "only-unify",
+				Usage: "stop after type unification",
+			},
 		}
 		result = append(result, runFlags...)
 	}
@@ -318,14 +323,25 @@ func (obj *GAPI) Cli(cliInfo *gapi.CliInfo) (*gapi.Deploy, error) {
 		}
 	}
 	logf("running type unification...")
+	startTime := time.Now()
 	unifier := &unification.Unifier{
 		AST:    iast,
 		Solver: unification.SimpleInvariantSolverLogger(unificationLogf),
 		Debug:  debug,
 		Logf:   unificationLogf,
 	}
-	if err := unifier.Unify(); err != nil {
+	unifyErr := unifier.Unify()
-		return nil, errwrap.Wrapf(err, "could not unify types")
+	delta := time.Since(startTime)
+	if unifyErr != nil {
+		if c.Bool("only-unify") {
+			logf("type unification failed after %s", delta)
+		}
+		return nil, errwrap.Wrapf(unifyErr, "could not unify types")
+	}
+
+	if c.Bool("only-unify") {
+		logf("type unification succeeded in %s", delta)
+		return nil, nil // we end early
 	}
 
 	// get the list of needed files (this is available after SetScope)

lib/run.go

@@ -79,6 +79,9 @@ func run(c *cli.Context, name string, gapiObj gapi.GAPI) error {
 	if err != nil {
 		return errwrap.Wrapf(err, "cli parse error")
 	}
+	if c.Bool("only-unify") && deploy == nil {
+		return nil // we end early
+	}
 	obj.Deploy = deploy
 	if obj.Deploy == nil {
 		// nobody activated, but we'll still watch the etcd deploy chan,