Add basic blocks and implementation of flattening thunks -> basic blocks

* Basic are a more linear way of representing code. Thunks beget basic
  blocks, which beget vectors of instructions.
* Basic blocks are also being flattened into a vector of instructions
  (hopefully, no tests done yet)
* OH yeah locals can be collected too (but currently are not being
  collected in the compiler, that should come soon)

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

src/vm/frame.rs

@@ -1,6 +1,5 @@
 use crate::obj::prelude::*;
 use shredder::{GcSafe, Scan, Scanner};
-use std::collections::BTreeMap;
 
 /// A stack call frame.
 #[derive(Debug, Clone)]
@@ -25,8 +24,6 @@ unsafe impl Scan for FrameKind {
 
 unsafe impl GcSafe for FrameKind {}
 
-pub type Locals = BTreeMap<usize, ObjRef>;
-
 #[derive(Scan, Debug, Clone)]
 pub struct Frame {
     locals: Locals,