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/mod.rs

@@ -254,6 +254,15 @@ impl<'c> Vm<'c> {
                     todo!("TODO: throw an error for attributes that can't be set");
                 }
             }
+            Inst::CheckTruth => {
+                let source = self.pop().expect("no source available for CheckTruth");
+                read_obj_downcast!(let obj: Option<&Bool> = &source);
+                if let Some(obj) = obj {
+                    self.condition = obj.value();
+                } else {
+                    unreachable!("Expected a boolean for CheckTruth but got {:?} instead", source);
+                }
+            }
             Inst::Jump(addr) => {
                 next_pc = addr;
             }
@@ -272,12 +281,12 @@ impl<'c> Vm<'c> {
                     .expect("TODO: throw an error for missing __call__ attr");
                 signal = Some(Signal::Call(callee, args));
             }
-            Inst::Index => todo!(),
+            Inst::Index => todo!("Inst::Index"),
             Inst::Return => {
                 signal = Some(Signal::Return);
             }
-            Inst::UnNeg => todo!(),
-            Inst::UnPos => todo!(),
+            Inst::UnNeg => todo!("Inst::UnNeg"),
+            Inst::UnPos => todo!("Inst::UnPos"),
             Inst::BinPlus => {
                 let rhs = self.pop().unwrap();
                 let lhs = self.pop().unwrap();