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'.