lang: unification: Improve type unification algorithm

The simple type unification algorithm suffered from some serious
performance and memory problems when used with certain code bases. This
adds some crucial optimizations that improve performance drastically.

lang/interfaces/ast.go

@@ -32,6 +32,7 @@ import (
 // often since we usually know which kind of node we want.
 type Node interface {
 	Apply(fn func(Node) error) error
+	//Parent() Node // TODO: should we implement this?
 }
 
 // Stmt represents a statement node in the language. A stmt could be a resource,