Cuddle can't parse .regexp controls

[jasagredo]

From the example in the RFC:

b.cddl:1:12:
  |
1 | nai = tstr .regexp "[A-Za-z0-9]+@[A-Za-z0-9]+(\\.[A-Za-z0-9]+)+"
  |            ^
unexpected '.'
expecting comment, end of input, name, or white space

[jasagredo]

Turns out it is caused by \\. in the regexp. But anyways that should be fixed.