libsecp256k1 allows overflowing signatures

[Kelvin Wong]

Refer to this: https://rustsec.org/advisories/RUSTSEC-2021-0076, should we need to fix this to upgrade to v0.5 or newer version ?

[Iraklis Leontiadis]

Refer to this: https://rustsec.org/advisories/RUSTSEC-2021-0076, should we need to fix this to upgrade to v0.5 or newer version ?

#171