Make TDL coreferences and symbols use same pattern as identifiers

[goodmami]

I updated the Identifier part of the _tdl_lex_re regex in delphin/tdl.py to follow the whitelist pattern at http://moin.delph-in.net/TdlRfc, but I did not change the pattern for coreferences or single-quoted symbols. These should all be the same. (But it might not be the simple whitelist we have now; discussion is still continuing on the mailing list... stay tuned).