Char generator

This can get enormously complicated, with latin1 in UTF-8 and non-Latin characters.
As a first implementation I propose ASCII (0-127) and printable ASCII (32-127) chars.
I first understood shrinking as a minimization of the complexity of the input, so shrinking `A` to `:` did not simplify the input very much. Due to the conventions we are adopting I will shrink towards the lowest code point to sort of "converge" failed tests towards a single input.