In this PR I remove all *Gadget types and instead implement the naming convention defined here #181 (comment)

The biggest changes can be summarised by:

• a) Removal of *Gadget types and their eval_* methods in favour of verb_object_circuit functions.
• b) The above functions don't create targets that need to be witness-assigned later. Instead they receive those as arguments. This clearly shows what's the circuit input and output.

I'm specially happy about the changes from b), I think they make the flow of data in the circuit more clear.

Missing things that I did not address in this PR

• The RecursiveCircuit still uses some old naming conventions like build.
• We have some *Target types that have methods that define constraints. I think we can keep those as they are convenient and I don't see them as strongly breaking the new convention: I see them as the object-oriented way to apply the convention. In those cases the object can be omitted from the method when it's implied by the type name, and the _circuit suffix doesn't appear because it's implied by the fact that the type is a *Target. Examples are: SignatureTarget::verify -> BoolTarget, StatementTarget::has_native_type -> BoolTarget or OperationTypeTarget::as_custom -> (BoolTarget, HashOutTarget, Target).