wip:

Add type system and refator for having only epression

tests/resolution.rs

@@ -37,13 +37,13 @@ fn resolved_nested_closure_reads_captured_local() {
     // `count` is read from inside a nested function, two scopes up; the
     // resolver records distance 2 and the interpreter uses `get_at(2, ..)`.
     let src = "\
-make :: fn () do
+make :: fn (): Any do
     count := 10;
-    get :: fn () do
+    get :: fn (): Number do
         return count;
-    end
+    end;
     return get;
-end
+end;
 g := make();
 g();
 ";