merkle tree structure needed for FRI

[agnxsh]

This Pull Request is related to #3 .

It changes the following:

• added the initial parent and leaf functions
• created a sha256 trait for hashing left and right child (need Poseidon with Stark252 params in it as well)
• added a ark PrimeField scaffold, (will need stark 252 fft friendly field later on)

I solumnly swear that I included unit tests: (will do, currently in draft WIP)

The PR is still a WIP, bugs/incomplete implementations may still be there