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/inst.rs

@@ -10,10 +10,16 @@ pub enum Inst {
     PushConst(ConstHandle),
 
     /// Looks up and pushes a local value.
-    LoadName(Name),
+    LoadLocal(Name),
+
+    /// Looks up and pushes a global value.
+    LoadGlobal(Name),
 
     /// Pop a value from the stack, possibly into a local symbol.
-    Pop(Option<Name>),
+    PopLocal(Option<Name>),
+
+    /// Pop a value from the stack, possibly into a global symbol.
+    PopGlobal(Option<Name>),
 
     /// Pops a symbol value and an object reference.
     ///
@@ -119,8 +125,10 @@ impl Inst {
         match self {
             Inst::PushSym(_) => "PUSH_SYM",
             Inst::PushConst(_) => "PUSH_CONST",
-            Inst::LoadName(_) => "LOAD_NAME",
-            Inst::Pop(_) => "POP",
+            Inst::LoadLocal(_) => "LOAD_LOCAL",
+            Inst::LoadGlobal(_) => "LOAD_GLOBAL",
+            Inst::PopLocal(_) => "POP_LOCAL",
+            Inst::PopGlobal(_) => "POP_GLOBAL",
             Inst::GetAttr(_) => "GET_ATTR",
             Inst::SetAttr(_) => "SET_ATTR",
             Inst::Jump(_) => "JUMP",