RUSTSEC-2021-0076 and libsecp256k1 version in sp-core

[trevor-crypto]

There has been a new RUSTSEC issue that stems from libsecp256k1: https://rustsec.org/advisories/RUSTSEC-2021-0076

I would like to bump libsecp256k1 in sp-core, but there are two blockers in the way:

1. There's a bug that is solved on master but not yet in a release, which is needed to make the changes from 0.3.5 -> 0.5.x: Removes redundant util::self libsecp256k1#63
2. libsecp256k1 no_std is broken. I submitted a PR already and tracking here: no-std support broken in 0.5.0 libsecp256k1#71

Once these are resolved I will submit a PR to bump the dependency and make the required changes.