- add -hidir flag to control the destination for .hi files when doing
  multiple compilations (matches -odir).

- change the ml_hi_file component of ModuleLocation from Maybe FilePath to
  FilePath.  We never made use of its Maybeness.

- clear out some unused code from the Hsc phase of run_phase and clean up
  a bit.  Fix a bug with the -ohi option at the same time (I don't think
  it works in 5.xx.x before this patch).