Introduce Original Mercurial Commitments

[nkaskov]

Implement the Mercurial Commitments together with the tests (it means you need to find an implementation or generate tests). Find the introduction to commitment schemes concept in the first section of the spec.

Is has to be compatible with the commitment scheme trait.

Commitments schemes are placed in here.

As a full example of already implemented commitment scheme LPC commitment can be used. You may need to use a bit different interface, but it makes sense to take a look at LPC.