Bytes and string generators

[pascutto]

Hi !
First of all thanks for this great tool !

I've been willing to use it over the past few days, and when I needed to generate bytes and strings, I realized:

• There is no bytes gen generator.
• val bytes : string gen sounds a bit confusing to me ; as I expected it either to be named string or to return a bytes gen.

Of course, one could easily use map to get a string gen out of a bytes gen, but it would also probably be easier to use and understand if Crowbar could provide generators for basic types, named accordingly.

So, here is my proposal (I'd be happy to open a PR for that):

• Rename bytes to string (since it returns a string gen).
• Add val bytes : bytes gen.
  These would obviously be breaking changes, so let me know what you think :)