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

We had added a necessary include directory to build.rs but not updated the
library on crates.io, so dependencies of rust-secp were failing to build.
(However, on my local system I had secp installed in /usr/local/include,
so I did not notice the problem until Travis pointed it out on a different
project!)