Implement support for custom seeds

[delcypher]

We need to implement support to give the fuzzer custom seeds.

Once that's implemented we need to implement the smart seeds. These seeds will be generated from a set of interesting values based on the free variable sort (e.g. for float NaN/+Inf/-Inf/+0/-0/1.0, smallest denormal, largest denormal, smallest normal, largest normal). Interesting values can also be pulled by looking for constants in the constraints themselves.