WIP: Capture return values during call sequence execution

[anishnaik]

Closes #505

Medusa will now capture the return values of every transaction executed (only the top-level call frame). The return value(s) must be of a primitive type (uint, int, bytes, fixed byte array, string, bool, address). These values are added to the value set during call sequence execution but are then wiped at the end of the call sequence. Thus, these values are ephemeral. This is to ensure that the value set does not blow up in size. The original value set (before the start of call sequence execution) is cached and is used to reset the call sequence.

The generation/mutation of fixed byte arrays has also been improved. Before, we always generated a completely random fixed byte array. Now, we use the value set, which should aid in finding bugs like #505. Additionally, we have reduced the probability of generating a completely random byte array/slice from 50% to 10%. It makes no sense to generate a completely random byte array/slice half the time.

Before merging we need to ensure that benchmarking shows that this does not cause immense performance degradation.