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/obj/boolean.rs

@@ -0,0 +1,92 @@
+use crate::{
+    obj::{prelude::*, reserved::*},
+    vm::error::*,
+};
+use once_cell::sync::Lazy;
+use shredder::Scan;
+use std::fmt::{self, Debug, Formatter};
+
+pub type BoolRef = ObjRef<Bool>;
+
+#[derive(Scan)]
+pub struct Bool {
+    value: bool,
+    vtable: Vtable,
+    attrs: Attrs,
+}
+
+impl Bool {
+    /// Gets the value of this boolean.
+    pub fn value(&self) -> bool {
+        self.value
+    }
+}
+
+impl From<bool> for BoolRef {
+    fn from(value: bool) -> Self {
+        if value {
+            BOOL_TRUE.clone()
+        } else {
+            BOOL_FALSE.clone()
+        }
+    }
+}
+
+impl_obj_readonly!(Bool);
+
+impl Debug for Bool {
+    fn fmt(&self, fmt: &mut Formatter) -> fmt::Result {
+        fmt.debug_struct("Bool").field("value", &self.value).finish()
+    }
+}
+
+/// A true boolean value.
+///
+/// This value should be the only instance of a "true" boolean.
+pub static BOOL_TRUE: Lazy<BoolRef> = Lazy::new(||
+    self_referring_obj! {
+        Bool {
+            value: true,
+            vtable: Default::default(),
+            attrs: Default::default(),
+        },
+        vtable: |obj_ref: ObjRef| vtable! {
+            BOOL_MEMBER_NAME.sym => Method::new_obj(obj_ref.clone(), BOOL_BOOL_FUN.clone()),
+        }
+    }
+);
+
+/// A false boolean value.
+///
+/// This value should be the only instance of a "false" boolean.
+pub static BOOL_FALSE: Lazy<BoolRef> = Lazy::new(||
+    self_referring_obj! {
+        Bool {
+            value: false,
+            vtable: Default::default(),
+            attrs: Default::default(),
+        },
+        vtable: |obj_ref: ObjRef| vtable! {
+            BOOL_MEMBER_NAME.sym => Method::new_obj(obj_ref.clone(), BOOL_BOOL_FUN.clone()),
+        }
+    }
+);
+
+////////////////////////////////////////////////////////////////////////////////
+// Builtin Bool functions
+////////////////////////////////////////////////////////////////////////////////
+
+/// Bool.__bool__(self) impl
+static BOOL_BOOL_FUN: Lazy<NativeFunRef> = Lazy::new(|| {
+    NativeFun::new_obj(1, |_callee, vm, args| {
+        {
+            read_obj_downcast!(let value: Option<&Bool> = &args[0]);
+            value.ok_or_else(|| Error::ValueError {
+                error: "expected Bool value".to_string(),
+            })?;
+        }
+        // just push a copy of the "self" value
+        vm.push(args[0].clone());
+        Ok(crate::vm::signal::Signal::Return)
+    })
+});