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/syn/mod.rs

@@ -4,4 +4,3 @@ pub mod lexer;
 pub mod parser;
 pub mod span;
 pub mod token;
-pub mod words;