The implementation is non-standard. Deprecation allows a simpler API in
the future without removing any features.

This commit removed the re2c lexer and replaced it by a manual version.
Its integration is not yet complete: number parsing does not respect
locales or overflows. Furthermore, parsing does not need to end with
EOF. Therefore, a lot of test cases fail. The idea is to push this
branch forward so we can conduct performance comparisons. So far, a
nice side effect are better diagnosis messages in case of parse errors.

Added class hierarchy for user-defined exceptions (#244). Integrated
parse exceptions 101-103. Parse exceptions include the byte count of
the last read character to locate the position of the error (#301).

BOMs currently yield an error.