Parse OPTIONS properly and cache the result.

Parse OPTIONS properly and cache the result.

Use the lexer to parse OPTIONS, LANGUAGE and INCLUDE pragmas.
This gives us greater flexibility and far better error
messages. However, I had to make a few quirks:
  * The token parser is written manually since Happy doesn't
    like lexer errors (we need to extract options before the
    buffer is passed through 'cpp'). Still better than
    manually parsing a String, though.
  * The StringBuffer API has been extended so files can be
    read in blocks.
I also made a new field in ModSummary called ms_hspp_opts
which stores the updated DynFlags. Oh, and I took the liberty
of moving 'getImports' into HeaderInfo together with
'getOptions'.