Add instruction dumper

A vector of instructions and constants can now be decompiled as text on
the terminal to give an idea of what the VM is doing.

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

src/compile/mod.rs

@@ -11,6 +11,7 @@ pub struct Compile {
     const_data: ConstData,
     globals: BTreeMap<Sym, Local>,
     locals: Vec<BTreeMap<Sym, Local>>,
+    names: Vec<Sym>,
     next_local: Local,
 }
 
@@ -63,6 +64,15 @@ impl Compile {
             .copied()
     }
 
+    /// Looks up a variable name, or creates a global name if it doesn't exist.
+    pub fn lookup_scope_or_create_global(&mut self, sym: Sym) -> Local {
+        if let Some(local) = self.lookup_scope(sym) {
+            local
+        } else {
+            self.create_global(sym)
+        }
+    }
+
     /// Creates a new local variable if it does not exist in the current local scope.
     pub(crate) fn create_local(&mut self, sym: Sym) -> Local {
         let locals = self.locals.last_mut().expect("scope");
@@ -73,6 +83,8 @@ impl Compile {
             let local = self.next_local;
             self.next_local = self.next_local.next();
             locals.insert(sym, local);
+            self.names.push(sym);
+            assert_eq!(self.names.len(), self.next_local.index());
             local
         }
     }
@@ -85,6 +97,8 @@ impl Compile {
             let global = self.next_local;
             self.next_local = self.next_local.next();
             self.globals.insert(sym, global);
+            self.names.push(sym);
+            assert_eq!(self.names.len(), self.next_local.index());
             global
         }
     }