DXR support
rebased
Fix bytepos_to_file_charpos.

Make bytepos_to_charpos relative to the start of the filemap rather than its previous behaviour which was to be realtive to the start of the codemap, but ignoring multi-byte chars in earlier filemaps. Rename to bytepos_to_file_charpos. Add tests for multi-byte chars.
span bugs
indexes again \o/
lots and lots