Add name-scanning pass to compiler

Compile::discover_locals now takes a Vec<SpExpr> instead of a single
SpExpr. It is also called before the compiler starts building the list
of instructions so that all names in the scope are available at
compile-time.

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

examples/factorial.sy

@@ -1,8 +1,5 @@
 [ :x x x ] :dup
 
-# This a dumb little workaround to allow for recursion so factorial inside of
-# the function definition will look in the correct scope
-0 :factorial
 [
     dup!
     [dup! 1 -! factorial! *!]

src/compile.rs

@@ -48,8 +48,8 @@ impl<'s> Compile<'s> {
         // Local scopes are implicit. If a variable is assigned to at
         // all in the current scope, it's considered to be a local.
         let mut code = Vec::new();
+        self.discover_locals(&expr_list);
         for expr in expr_list {
-            self.discover_locals(&expr);
             code.extend(self.compile_expr(&expr));
         }
         code
@@ -102,11 +102,13 @@ impl<'s> Compile<'s> {
     ///
     /// If any assignment for a local variable appears in the scope, it will be
     /// inserted.
-    fn discover_locals(&mut self, expr: &SpExpr) {
+    fn discover_locals(&mut self, exprs: &Vec<SpExpr>) {
+        for expr in exprs.iter() {
             if let Expr::Atom(atom) = expr.inner() {
                 if let Atom::Assign(text) = atom.inner() {
                     self.scope_stack.insert_local(text);
                 }
             }
         }
+    }
 }