Implement GCID parsing in unicsv and extend ID to 64bit (#501)

Implement GCID parsing in unicsv and extend ID to 64bit (#501)

* Implement GCID parsing in unicsv and extend ID to 64bit

This reimplements the parser and adds some test cases. The test
cases have been validated with several online converters, so this
hopefully works better than the previous implementation.

This also fixes an endless loop that afl managed to trigger.

* Fix clazy warning in unicsv

* Add GC-ID test cases, documentation, int parsing

- Explicitly do the integer parsing with base 10, otherwise hex
  values and octal values would be accepted with 0x and 0 prefix.
- Add testcases that cover GC-ID parsing, including some invalid cases
- Unify the terminology (GC-Code vs GC-ID)
- Add examples to the unicsv documentation showing that both GC-ID
  and GC-Code are accepted