added possibility to choose random number generator explicitly from outside

[LordBlackhawk]

Sometimes it is important to choose the random number generator explicitly from outside:

• reproducable sudoku generation
• compiling sudoku to web assembly target, since rand::thread_rng does not work there

[Emerentius]

This is a good feature to have. I don't want to duplicate all generate_* functions for an explicit rng though, so I'm going to cut it down to only the minimal 2 functions needed for full generality: generate_solved_with_rng and generate_with_symmetry_and_rng_from.

I might swap sudoku generation to a builder style API in the future to curb the number of methods without sacrificing flexibility and conciseness.