Teach `DiscretePDF` to take a comparison function that is used to do

comparison on the key type. The purpose of doing is to avoid doing
comparisons on the pointer address of `ExecutionState*` which is not
stable between KLEE versions or with ASLR. Instead we use the
`ExecutionState`'s unique ID.

This is base commit that both the Aachen and Imperial tools will be

rebased on.

Fix really stupid bug in dynamic solver.

Because we do max(v, 0.0) where `v` is some value we may set the
solver timeout to 0.0. The semantics of this in KLEE actually mean
run with **NO TIMEOUT** which is not what we want. What we actually
want is for the solver to basically to fail very quickly.

This now "sort of" fixed by actually just avoiding solver invocation
completely when this scenario occurs.

Tool exchange version of tool but rebased

on newer KLEE revision (2852ef6)
+ dynamic solver timeout version 2.

Commit used for tool exchange with Aachen.

Commit of klee-afr used when exchanging benchmarks with Aachen.