Add fuzzing targets

[Robbepop]

Stevia could profit greatly from fuzz testing.

This could be more or less easily implemented at least for SMTLib2 parsing once implemented and for the word-level simplifier. Maybe later even for the bit blaster module.

For this there is a tool called cargo-fuzz that should work out of the box but we need parsing support for SMTLib2 before we can do any proper fuzz testing at all.