lang: Add modern type unification implementation

This adds a modern type unification algorithm, which drastically
improves performance, particularly for bigger programs.

This required a change to the AST to add TypeCheck methods (for Stmt)
and Infer/Check methods (for Expr). This also changed how the functions
express their invariants, and as a result this was changed as well.

This greatly improves the way we express these invariants, and as a
result it makes adding new polymorphic functions significantly easier.

This also makes error output for the user a lot better in pretty much
all scenarios.

The one downside of this patch is that a good chunk of it is merged in
this giant single commit since it was hard to do it step-wise. That's
not the end of the world.

This couldn't be done without the guidance of Sam who helped me in
explaining, debugging, and writing all the sneaky algorithmic parts and
much more. Thanks again Sam!

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

lang/gapi/gapi.go

@@ -277,9 +277,7 @@ func (obj *GAPI) Cli(info *gapi.Info) (*gapi.Deploy, error) {
 	if !args.SkipUnify {
 		// apply type unification
 		unificationLogf := func(format string, v ...interface{}) {
-			if debug { // unification only has debug messages...
-				logf("unification: "+format, v...)
-			}
+			logf("unification: "+format, v...)
 		}
 		logf("running type unification...")