MD6 Hash(Pure Rustlang)

Paper: https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=c03d3c666be3cdd6107ee695cbaa47cd7305c4b7

TODO:

• refactor the hash function similar to c++ version ✔️
• implement the no_std version of md6 ✔️
• implement the digest trait (similar to md5) ✔️
• add the testing for no_std version ✔️