Log Message:
Add security/libsecp256k1

This is required for newer versions of finance/electrum.

Optimized C library for ECDSA signatures and secret/public key operations on
curve secp256k1.

This library is intended to be the highest quality publicly available library
for cryptography on the secp256k1 curve. However, the primary focus of its
development has been for usage in the Bitcoin system and usage unlike Bitcoin's
may be less well tested, verified, or suffer from a less well thought out
interface. Correct usage requires some care and consideration that the library
is fit for your application's purpose.

Features:
* secp256k1 ECDSA signing/verification and key generation.
* Additive and multiplicative tweaking of secret/public keys.
* Serialization/parsing of secret keys, public keys, signatures.
* Constant time, constant memory access signing and public key generation.
* Derandomized ECDSA (via RFC6979 or with a caller provided function.)
* Very efficient implementation.
* Suitable for embedded systems.
* Optional module for public key recovery.
* Optional module for ECDH key exchange.

Experimental features have not received enough scrutiny to satisfy the standard
of quality of this library but are made available for testing and review by the
community. The APIs of these features should not be considered stable.