Replace secp256k1 with libsecp256k1 pure rust implementation

[Gauthamastro]

PR replaces secp256k1 with libsecp256k1 pure implementation from paritytech.

[elichai]

I'll link these 2 issues again: #44 and #43, and I recommend you read them before :)
I'll quote what I wrote in #32:

I would prefer https://github.com/RustCrypto/elliptic-curves/tree/master/k256 or something based on fiat-crypto.

but even then it's quite scary, as libsecp256k1 is very battle tested.
another option would be to fork libsecp256k1 and write bindings to the internal functionality, that allows infinity, jacobian points and more.

As for the SGX problems, I used libsecp256k1 in SGX in the past, and worked a lot to make rust-secp256k1 work in virtually every environment possible (See rust-bitcoin/rust-secp256k1#100, rust-bitcoin/rust-secp256k1#115, rust-bitcoin/rust-secp256k1#125, rust-bitcoin/rust-secp256k1#130)
So if you can show me what's not working for you I'd love to try and help (I can also do that privately in Telegram/IRC/Signal if you prefer)

[Gauthamastro]

Closing this as per our discussion in telegram.