build secp256 on android

Added methods for secretstore implementation

Add curve points (combine public keys) exposed

Andrew Poelstra authored Aug 07, 2016 and Arkadiy Paronyan committed Aug 20, 2016

Turns out you cannot initialize constant SecretKeys in any way; these
two constants should cover most sane use cases (other good choices
are the SECG generator and the Alpha CT generator, but these will
wait for a major CT-supporting upgrade, unless demand for them appears.)

merge with apoelstra/master

cleanup

add #[repr(C)] to SecretKey, SharedSecret

Bump libc

Bumping libc

Removed unused attributes