[dep] bump generic-array to 0.12.4
[dep] bump generic-array to 0.12.4

This addresses RUSTSEC-2020-0146.

Closes: #7819

libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra