Replaced libsecp256k1 with rust-secp256k1

[elichai]

rust-secp256k1 now has wasm support. It has a lot of advantages over the libsecp library:

1. Promise side channel resistance.
2. Has multiple writers and gone through a lot of reviewers.
3. Really fast

https://github.com/rust-bitcoin/rust-secp256k1/pull/125#issuecomment-508598506