Finalize moves-based-on-type implementation.

Changes:

- Refactor move mode computation
- Removes move mode arguments, unary move, capture clauses
  (though they still parse for backwards compatibility)
- Simplify how moves are handled in trans
- Fix a number of illegal copies that cropped up
- Workaround for bug involving def-ids in params
  (see details below)

Future work (I'll open bugs for these...):

- Improve error messages for moves that are due
  to bindings
- Add support for moving owned content like a.b.c
  to borrow check, test in trans (but I think it'll
  "just work")
- Proper fix for def-ids in params

Def ids in params:

Move captures into a map instead of recomputing.

This is a workaround for a larger bug having to do with the def-ids associated
with ty_params, which are not always properly preserved when inlining.  I am
not sure of my preferred fix for the larger bug yet.  This current fix removes
the only code in trans that I know of which relies on ty_param def-ids, but
feels fragile.