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/vm/inst.rs

@@ -9,7 +9,7 @@ pub enum Inst {
     PushConst(ConstHandle),
 
     /// Looks up and pushes a local value.
-    PushLocal(Sym),
+    PushLocal(Local),
 
     /// Pop a value from the stack, possibly into a local symbol.
     Pop(Option<Sym>),