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/mod.rs

@@ -81,8 +81,6 @@ impl<'c> Vm<'c> {
     /// Set the next program counter value.
     ///
     /// This may cause the running program to crash. Handle with care.
-    ///
-    /// TODO : consider making this `unsafe`? Is that appropriate in this context?
     pub fn set_pc(&mut self, pc: usize) {
         self.pc = pc;
     }