Add Bool builtin type, branches, and some more stuff

* obj::Bool builtin type is used for truthiness and decision-making
* Branches are compiled and seem to be working for basic integer
  comparison
* Updated version of Shredder to what is current as of writing
* CheckTruth VM instruction that will explicitly set the condition flag
* Probably some other stuff

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

src/vm/inst.rs

@@ -1,5 +1,4 @@
 use crate::{obj::prelude::*, vm::consts::*};
-use shredder::EmptyScan;
 
 #[derive(Debug, PartialEq, Clone, Copy)]
 pub enum Inst {
@@ -36,6 +35,9 @@ pub enum Inst {
     ///
     SetAttr(Sym),
 
+    /// Pops the top value off of the stack and checks if it is a `Bool` of a true value.
+    CheckTruth,
+
     /// Jump to a given address in the current function unconditionally.
     Jump(usize),
 
@@ -131,6 +133,7 @@ impl Inst {
             Inst::PopGlobal(_) => "POP_GLOBAL",
             Inst::GetAttr(_) => "GET_ATTR",
             Inst::SetAttr(_) => "SET_ATTR",
+            Inst::CheckTruth => "CHECK_TRUTH",
             Inst::Jump(_) => "JUMP",
             Inst::JumpTrue(_) => "JUMP_TRUE",
             Inst::Call(_) => "CALL",
@@ -154,4 +157,4 @@ impl Inst {
     }
 }
 
-impl EmptyScan for Inst {}
+unsafe impl shredder::marker::GcDrop for Inst {}