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/vm/error.rs

@@ -13,8 +13,6 @@ pub enum RuntimeError {
 
     #[error("cannot call non-quote value '{0}'")]
     CannotCall(String),
-    //#[error("stack underflow")]
-    //StackUnderflow,
 }
 
 pub type Result<T, E = RuntimeError> = std::result::Result<T, E>;

src/vm/inst.rs

@@ -1,5 +1,5 @@
 use crate::object::Value;
-use crate::syn::words::Word;
+use crate::scope::Word;
 
 #[derive(Debug, Clone)]
 pub enum Inst {

src/vm/machine.rs

@@ -1,6 +1,7 @@
 use crate::compile::Compile;
 use crate::object::*;
-use crate::syn::{ast::SpExpr, words::*};
+use crate::scope::*;
+use crate::syn::ast::SpExpr;
 use crate::vm::{error::*, inst::*};
 use std::cell::RefCell;
 use std::collections::BTreeMap;