lang: parser, ast, interfaces: Implement include as

This adds support for `include as <identifier>` type statements which in
addition to pulling in any defined resources, it also makes the contents
of the scope of the class available to the scope of the include
statement, but prefixed by the identifier specified.

This makes passing data between scopes much more powerful, and it also
allows classes to return useful classes for subsequent use.

This also improves the SetScope procedure and adds to the Ordering
stage. It's unclear if the current Ordering stage can handle all code,
or if there exist corner-cases which are valid code, but which would
produce a wrong or imprecise topological sort.

Some extraneous scoping bugs still exist, which expose certain variables
that we should not depend on in future code.

Co-authored-by: Samuel Gélineau <gelisam@gmail.com>

lang/interfaces/const.go

@@ -28,6 +28,12 @@ const (
 	// also used with `ModuleSep` for scoped variables like `$foo.bar.baz`.
 	VarPrefix = "$"
 
+	// BareSymbol is the character used primarily for imports to specify
+	// that we want to import the entire contents and flatten them into our
+	// current scope. It should probably be removed entirely to force
+	// explicit imports.
+	BareSymbol = "*"
+
 	// PanicResKind is the kind string used for the panic resource.
 	PanicResKind = "_panic"
 )