lang: ast, parser, interfaces: Implementation of nested class sugar

This implements a new type of syntactic sugar for the common pattern of
a base class which returns a child class, and so on. Instead of needing
to repeatedly indent the child classes, we can instead prefix them at
the definition site (where created with the class keyword) with the name
of the parent class, followed by a colon, to get the desired embedded
sugar.

For example, instead of writing:

class base() {
	class inner() {
		class deepest() {
		}
	}
}

You can instead write:

class base() {
}
class base:inner() {
}
class base:inner:deepest() {
}

Of course, you can only access any of the inner classes by first
including (with the include keyword) a parent class, and then
subsequently including the inner one.

lang/interfaces/const.go

@@ -23,6 +23,10 @@ const (
 	// It is also used for variable scope separation such as `$foo.bar.baz`.
 	ModuleSep = "."
 
+	// ClassSep is the character used for the class embedding separation.
+	// For example when defining `class base:inner` this is the char used.
+	ClassSep = ":"
+
 	// VarPrefix is the prefix character that precedes the variables
 	// identifier. For example, `$foo` or for a lambda, `$fn(42)`. It is
 	// also used with `ModuleSep` for scoped variables like `$foo.bar.baz`.