Move TestConstraintSystem out of sapling-crypto

[str4d]

TestConstraintSystem is a useful module for testing circuit code, that currently lives inside the sapling-crypto crate alongside the Sprout and Sapling circuits. It should be moved somewhere that is obviously part of the toolkit for circuit implementers.

This will probably be a new crate that depends on bellman (given that @ebfull wants bellman to be flexible enough that such things don't need to be declared or maintained by bellman itself).

Part of #41.

[str4d]

Maybe a new crate named bellman_toolkit or bellman_testing?

[str4d]

#104 moves TestConstraintSystem into the gadgets module of bellman. This is sufficient for now; additional refactoring can happen when bellman itself is refactored.