Have source as match surface for chunks?

[mr-martian]

As discussed in #59, it would make sense to have the source be used for matching for both LUs and chunks. However, there are several difficulties with this.

• Chunks matching on target is baked in as an assumption in a lot of different places.
• This would introduce an incompatibility with existing bytecode file.
• When generating bytecode, neither compiler pays attention to whether the contents of a particular chunk are themselves chunks or not, so it's unclear whether generated output should
  • have a source side and then rtx-proc can just output the source if there's no target
  • have a target side
  • or we could come up with a syntax for specifying both