[hex-literal] Add new hex_custom_padding!() macro; enables custom padding between hex characters

[mcronce]

Specifically, my use case is using key hashes from Android signing key reports, which are colon-delimited (e.g. 55:1A:14:14:BA:03:DA:1B:23:ED:EA:40:4A:BE:88:D7:C4:83:D1:E2:85:42:41:2C:22:73:D3:52:0B:6E:A7:5F). Removing the colons before use would work, but keeping them matching the signing key in source code provides a lot of value in terms of maintainability

Implemented by making the existing hex!() macro a wrapper around this, with no padding set

Probably needs docs; as long as this seems like something the community is comfortable with, I'll add a doc comment to the macro

[newpavlov]

Hm. Maybe it would be better to just add : to the list of ignored characters? @tarcieri WDYT?

[tarcieri]

Sure, seems fine

[newpavlov]

Closing in favor of #1244.

[mcronce]

Thanks!