Remove redundancy in filenames

[AntoineRondelet]

See: clearmatics/libsnark#35

An example in Zeth is: libzeth/snarks/groth16/groth16_api_handler.hpp that can be changed to libzeth/snarks/groth16/api_handler.hpp etc.

[AntoineRondelet]

Add an entry about that in the CODING_STANDARDS.md: https://github.com/clearmatics/zeth/blob/master/CODING_STANDARDS.md