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/mod.rs

@@ -3,7 +3,7 @@ pub mod error;
 mod locals;
 pub mod thunk;
 
-use crate::{obj::prelude::*, vm::consts::*};
+use crate::{syn::ast::Body, obj::prelude::*, vm::consts::*};
 use std::collections::{BTreeMap, HashMap};
 
 pub struct Compile<'t> {
@@ -96,6 +96,11 @@ impl<'t> Compile<'t> {
     pub fn pop_scope_layer(&mut self) -> Option<BTreeMap<Sym, Local>> {
         self.locals.pop()
     }
+
+    /// Collects local variables for the given AST body non-recursively.
+    pub fn collect_locals(&mut self, body: &Body) {
+        locals::CollectLocals::new(self).collect(body);
+    }
 }
 
 #[derive(Debug, Default)]