Objs replace Values

* Obj is now a trait instead of a struct
* Value enum is gone, replaced with individual structs that implement
  Obj
* All instances where a Value was used, a Gc<(dyn Obj + 'static)> takes
  its place. ObjPtr is shorthand for this.
* A __str__ method for objects is implemented for a couple builtin types
  as a proof-of-concept
* println and print builtin functions are implemented using this
  interface

There's still a lot to do, mostly with interned values. For example,
whenever a new string object is created, a new __str__ function object
is created each time, rather than reusing the first one that was
created. Stuff like that.

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

src/compile.rs

@@ -1,4 +1,4 @@
-use crate::object::{QuoteTable, Value};
+use crate::object::*;
 use crate::scope::*;
 use crate::syn::ast::*;
 use crate::vm::inst::*;
@@ -66,7 +66,7 @@ impl<'s> Compile<'s> {
                 let quote =
                     self.quote_table
                         .insert(expr.span().clone(), locals, stmts.clone(), compiled);
-                let inst = Inst::PushValue(Value::Quote(quote));
+                let inst = Inst::PushValue(QuoteObj::new(quote).into_gc());
                 vec![SpInst::new(expr.span().clone(), inst)]
             }
         }
@@ -74,9 +74,9 @@ impl<'s> Compile<'s> {
 
     fn compile_atom(&mut self, atom: &SpAtom) -> Vec<SpInst> {
         let inst = match atom.inner() {
-            Atom::Float(f) => Inst::PushValue(Value::Float(*f)),
-            Atom::Int(i) => Inst::PushValue(Value::Int(*i)),
-            Atom::Str(s) => Inst::PushValue(Value::Str(s.clone())),
+            Atom::Float(f) => Inst::PushValue(FloatObj::new(*f).into_gc()),
+            Atom::Int(i) => Inst::PushValue(IntObj::new(*i).into_gc()),
+            Atom::Str(s) => Inst::PushValue(StrObj::new(s.clone()).into_gc()),
             Atom::Assign(text) => {
                 let word = self.scope_stack.insert_local(text);
                 Inst::Store(word)