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/object.rs

@@ -4,13 +4,11 @@
 #![allow(dead_code)]
 
 use crate::syn::ast::SpExpr;
-use crate::{
-    syn::{span::Span, words::Scope},
-    vm::inst::Inst,
-};
+use crate::vm::machine::Machine;
+use crate::{scope::Scope, syn::span::Span, vm::inst::Inst};
 use std::cell::RefCell;
 use std::collections::{BTreeMap, HashMap};
-use std::fmt::{self, Display};
+use std::fmt::{self, Debug, Display};
 use std::rc::{Rc, Weak};
 
 pub type Str = String;
@@ -72,6 +70,7 @@ pub enum Value {
     Str(Str),
     Quote(Quote),
     ObjPtr(ObjPtr),
+    BuiltinFn(BuiltinFn),
 }
 
 impl Value {
@@ -84,6 +83,7 @@ impl Value {
             Str(_) => "str",
             Quote(_) => "quote",
             ObjPtr(_) => "object",
+            BuiltinFn(_) => "builtin function",
         }
     }
 }
@@ -98,10 +98,24 @@ impl Display for Value {
             Str(s) => write!(fmt, "{}", s),
             Quote(q) => write!(fmt, "[quoted value #{}]", q.index()),
             ObjPtr(o) => write!(fmt, "[object #{}]", o.slot()),
+            BuiltinFn(b) => write!(fmt, "[{:?}]", b),
         }
     }
 }
 
+#[derive(Clone)]
+pub struct BuiltinFn(Rc<dyn FnMut(&mut Machine)>);
+
+impl Debug for BuiltinFn {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        write!(
+            fmt,
+            "builtin function at {:#x}",
+            (&self.0 as *const _ as usize)
+        )
+    }
+}
+
 // /////////////////////////////////////////////////////////////////////////////
 // Obj
 // /////////////////////////////////////////////////////////////////////////////