Upgrade secp256k1 library

[chfast]

Upgrade secp256k1 library to commit https://github.com/bitcoin-core/secp256k1/commit/9d560f992db26612ce2630b194aef5f44d63a530.

• [x] Check what functions from library are used and check theirs test coverage (we don't require them to be tested directly, it is enough that the API wrapper in form of devcrypto library has good coverage). If test should be extended or improved, please do this before upgrading the secp256k1 source code.
• [ ] We have a copy of the old version of secp256k1 source code. Upgrade it to the given commit and update the contact points (the API has changed since then).
• [ ] Configure the secp256k1 not to use GMP.
• [ ] Remove GMP dependency from cpp-ethereum.

[chfast]

Checked: every function from secp256k1 we use is at least touched by unit tests.