Update how scope rules work, and update implementation

* Currently, scopes are only allowed to look at their locals and the
  globals. Inner functions cannot refer to values in their parent
  functions. This will change eventually.
* Scope lookup is split between globals and locals. Locals are defined
  in a scope if they are explicitly assigned to.
    * i.e. `a = foo` will treat `a` as a local in the current scope if
      it appears anywhere in that scope. This does not extend to
      setattrs; `a.b = foo` will not trigger `a` into being a local var.
* `Package` objects are no longer returned from the compiler - instead,
  a user function is returned.
* Other various changes and renames

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

src/vm/frame.rs

@@ -1,5 +1,6 @@
 use crate::obj::prelude::*;
 use shredder::{GcSafe, Scan, Scanner};
+use std::collections::BTreeMap;
 
 /// A stack call frame.
 #[derive(Debug, Clone)]
@@ -25,20 +26,20 @@ unsafe impl GcSafe for FrameKind {}
 
 #[derive(Scan, Debug, Clone)]
 pub struct Frame {
-    locals: Names,
+    locals: FrameLocals,
     kind: FrameKind,
 }
 
 impl Frame {
-    pub fn new(locals: Names, kind: FrameKind) -> Self {
+    pub fn new(locals: FrameLocals, kind: FrameKind) -> Self {
         Self { locals, kind, }
     }
 
-    pub fn locals(&self) -> &Names {
+    pub fn locals(&self) -> &FrameLocals {
         &self.locals
     }
 
-    pub fn locals_mut(&mut self) -> &mut Names {
+    pub fn locals_mut(&mut self) -> &mut FrameLocals {
         &mut self.locals
     }
     
@@ -46,3 +47,5 @@ impl Frame {
         &self.kind
     }
 }
+
+pub type FrameLocals = BTreeMap<usize, ObjRef>;