Move src/syn/word.rs to src/scope.rs; add BuiltinFn

* crate::scope instead of crate::syn::word, which makes more sense
  because scopes are not really used in syntax at all
* Reference-counted BuiltinFn values are available for creation now

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

src/compile.rs

@@ -1,5 +1,6 @@
 use crate::object::{QuoteTable, Value};
-use crate::syn::{ast::*, words::*};
+use crate::scope::*;
+use crate::syn::ast::*;
 use crate::vm::inst::*;
 
 pub struct Compile<'s> {