Add configuration in a monadic way, for now used only to configure SMT2 debugging

[msooseth]

Description

This is a rather big change. It adds a monad that allows us to pass a global configuration down. This can be extremely helpful, e.g. also for abstraction-refinement, Expr debugging, and a bunch of other things. It required a bit of change here-and-there, but it's not that bad. It will allow us to pass a lot less parameters around for global configs.

This also adds dumping of SMT2 queries. This is done by adding one more field to the SMT2 datatype, the Expr that it was generated from. This is essential, otherwise we don't know from that place the SMT2 is coming from. It helps significantly with debugging the SMT2. Passing this Expr in any other way is not really possible, not in a clean way as far as I can see. It really does belong with the SMT2, I think.

Configurations supported:

• SMT, Expression, Prop dumping
• abstraction-refinement (revert 3b05c85 in case it's undesired)

Checklist

• tested locally
• added automated tests
• updated the docs
• updated the changelog

[msooseth]

Note, as part of the Keccak PR I am making, I am moving more things to the Config. So let's hold off a bit on adding more things to the Config on this PR. I promise to extend Config with more options as part of the Keccak PR here: #382

[d-xo]

just a few small nits, but otherwise looks great!

cc @arcz this is a breaking change that may impact echidna.