Properly scope local let in trait default implementations

When a trait is applied to a type, we copy over the method body
to the new type and as part of this scope everything. This works
fine for `var` which can be redefined at will. Let is a different
story. Given that we already scoped and defined our let, when we go
to do this on the actual type, we end up with an error that we are
redefining it. What we want to be doing is to set local to undefined
and allow them to be set to defined when the expression type checking
happens in the new type.

Closes ponylang#684

minimal-cases/issue-684/684.pony

@@ -0,0 +1,9 @@
+trait T
+  fun foo() =>
+    let x: U32 = 0
+
+class Baz is T
+class Bar is T
+
+actor Main
+  new create(env: Env) => None

src/libponyc/pass/traits.c

@@ -442,6 +442,13 @@ static ast_result_t rescope(ast_t** astp, pass_opt_t* options)
 
     case TK_LET:
     case TK_VAR:
+    {
+      ast_t* scope = ast_parent(ast);
+      ast_t* id = ast_child(ast);
+      ast_set(scope, ast_name(id), id, SYM_UNDEFINED);
+      break;
+    }
+
     case TK_MATCH_CAPTURE:
     {
       ast_t* scope = ast_parent(ast);