update mcl/bls to the latest version

[herumi]

• It is better to define MCL_MAX_BIT_SIZE=256 if only BN254 is used and so libbls384.a is renamed to libbls256.a and so on.
• define MCLBN_FP_UNIT_SIZE=4 before including bls.h.

[infinity0]

We recently updated to a monorepo and I think together with this PR this repo will become obsolete.

Please see the alternative suggestion in https://github.com/dfinity-lab/dev/issues/116 (we will work on that)