Add local variable collection to compilation

* Upon entering a new body, the local scope variables are collected.
* VM instruction PushLocal now uses a `Local` value instead of a `Sym`
  value
* Compilation of a new AST body will implicitly create a new scope

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/compile/locals.rs

@@ -50,7 +50,3 @@ impl Visit for CollectLocals<'_, '_> {
     fn visit_access_expr(&mut self, expr: &AccessExpr) -> Self::Out { DefaultAccept::default_accept(expr, self); }
     fn visit_atom(&mut self, atom: &Atom) -> Self::Out { DefaultAccept::default_accept(atom, self); }
 }
-
-pub fn collect_locals(compile: &mut Compile, body: &Body) {
-    CollectLocals::new(compile).collect(body);
-}