Merge #678: Preventing compiler optimizations in benchmarks without a memory fence

362bb25608dbcd724a07dd5170c4ebe081c3dd84 Modified bench_scalar_split so it won't get optimized out (Elichai Turkel)
73a30c6b58f078b42a03a222c55bfe8b4dd86a2b Added accumulators and checks on benchmarks so they won't get optimized out (Elichai Turkel)

Pull request description:

  As asked https://github.com/bitcoin-core/secp256k1/pull/667#issuecomment-546885951 this is the parts of #667 that don't require an assembly memory fence.

  I splitted them to 2 commits, one with obvious easy ones. and another that changes the logic a bit to achieve this (See https://github.com/bitcoin-core/secp256k1/pull/667#discussion_r337248398 )

ACKs for top commit:
  jonasnick:
    ACK 362bb256
  real-or-random:
    ACK 362bb25608dbcd724a07dd5170c4ebe081c3dd84 I read the diff and I ran the benchmarks

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