semantic+main: add the BuiltinContext and use it as semantic root

compiler/__main__.py

@@ -43,7 +43,7 @@ def main():
         ast = parser.parse()
         ast.pprint(depth=10)
 
-        context = semantic.Context("root")
+        context = semantic.BuiltinContext()
         ast.semantic_analysis(context)
 
         intermediate_representation = ast.intermediate_representation()

compiler/semantic.py

@@ -131,3 +131,12 @@ class Context:
 
     def __repr__(self) -> str:
         return self._pprint()
+
+
+class BuiltinContext(Context):
+    def __init__(self):
+        super().__init__(name="builtins", parent=None)
+        self.types = {
+            "__unknown": Type(self, "__unknown", "builtin"),
+            "uint32": Type(self, "uint32", "builtin")
+        }