lang: funcs: Add deploy package with readfile related functions

This adds a readfile function to actually access files from our deploy.
A fun side effect is that we can even access our own code! In general,
it's a good reminder that you should only run trusted code on your own
infrastructure. This also includes a fancy new test case.

lang/funcs/core/os/readfile_func.go

@@ -35,6 +35,8 @@ func init() {
 
 // ReadFileFunc is a function that reads the full contents from a local file. If
 // the file contents change or the file path changes, a new string will be sent.
+// Please note that this is different from the readfile function in the deploy
+// package.
 type ReadFileFunc struct {
 	init *interfaces.Init
 	last types.Value // last value received to use for diff