Implement macro to generate Parameters

[howardwu]

We probably should write a macro which impls Parameters, which would be invoked like so:

impl_params!(AccountCommitmentParameters, "./account_commitment.checksum", 417868, "./account_commitment.params");

this would reduce the LoC of the parameters module and make it easier to maintain / review when changes are needed.

In that case I can see us removing all the .rs files in the module and just calling the impl macro in the lib.rs file. Then, we could also move all the .params and .sum files to a subdirectory called params (or something)

Originally posted by @gakonst in https://github.com/AleoHQ/snarkOS/pull/184