Merge #835: Don't use reserved identifiers memczero and benchmark_verify_t

1f4dd0383807bfb7fef884601357b4c629dfb566 Typedef (u)int128_t only when they're not provided by the compiler (Tim Ruffing)
e89278f211a526062745c391d48a7baf782b4b2b Don't use reserved identifiers memczero and benchmark_verify_t (Tim Ruffing)

Pull request description:

  As identified in #829 and #833. Fixes #829.

  Since we touch this anyway, this commit additionally makes the
  identifiers in the benchmark files a little bit more consistent.

  This is necessary before we can merge #833. I preferred a separate PR because it makes it easier to see the results of Travis in #833.

ACKs for top commit:
  sipa:
    utACK 1f4dd0383807bfb7fef884601357b4c629dfb566
  jonasnick:
    ACK 1f4dd0383807bfb7fef884601357b4c629dfb566

Tree-SHA512: c0ec92798f3c94f3ef6ac69b3f0f39a39257a32be9d9a068832cece1ebe64c89848b70e44652fc397004b8b240883ac4bc0c8f95abbe4ba4b028de120e6734bf

diff --cc src/secp256k1.c
Simple merge