parametirize string shrinker with a char shrinker

[n-osborne]

This PR is an attempt to fix #257.

It does so by:

• adding an optional argument to Shrink.string for character shrinking, with a conservative default
• fix character shrinkers in QCheck.{printable_string,printable_string_of_size,small_printable_string,numeral_string,numeral_string_of_size} so that they have the expected behaviour.

For now, we don't expose a string arbitrary that takes a shrinker as argument. Albeit it would be nice, there is the question of how to do it. I see three possibilities:

1. add an optional argument to QCheck.string_gen and QCheck.string_gen_of_size
2. add a non-optional argument to QCheck.string_gen and QCheck.string_gen_of_size
3. add two functions: QCheck.string_gen_shrink and QCheck.string_gen_shrink_of_size

[jmid]

Short and sweet - LGTM - thanks! :pray: