Support CRLF BLIF files and expose string parser

[benjaminwp18]

On Windows, YOSYS generates BLIF files with CRLF line endings. blif-parser currently errors on CRLF line endings. This PR:

• makes blif-parser accept both \n and \r\n
• makes the string parsing function public so any future file format wonkiness can be solved by the user

I'd be happy to make changes if anyone sees this and has feedback!

[joonho3020]

It is undesirable to have global constants embedded in the file.

[benjaminwp18]

Replaced with local constants; only used in one function

[joonho3020]

• The changes you made causes the parser to hang for certain files. Can you investigate the issue and fix it? The test is added in the upstream blif-parser repo

[benjaminwp18]

Looks like the hang was just very slow parsing; removing my take_until_newline parser brought performance back in line with the trunk. take_until_newline used an alt that performed MUCH worse than a single take_until("\n"), and all its usages discarded the \r separately anyway.