parser-combinators

It would be nice to add the bits of code from chapter 33 of "Programming in Scala" as tests to be sure we don't break compatibility (too much) with the book.

I'm trying out lexy with a re-implementation of a parser I've already got which, in part, parses register names of the form r<0..31> and f<0..15> - i.e. there are 32 general purpose registers (named r0, r1, ... r31) and 16 floating point registers.

I'd like to verify that the register number is in range when parsed, so I'd like to have something like lexy::code_point, which has tr