Merge #662: Add ecmult_gen, ecmult_const and ecmult to benchmark

8f879c2887e166da2ec959ce78078f7b84ebfdf9 Fix array size in bench_ecmult (Jonas Nick)
2fe1b50df16c9f41ea77b151634d734b930eeddd Add ecmult_gen, ecmult_const and ecmult to benchmark (Jonas Nick)
593e6bad9c5cda05dd72a5bd8266c4880113b4af Clean up ecmult_bench to make space for more benchmarks (Jonas Nick)

Pull request description:

  I was trying to determine the impact of ecmult_gen in schnorrsig signing and noticed that there is no way to bench this right now. The new benchmarks look like this:
  ```
  $ ./bench_ecmult
  ecmult_gen: min 20.9us / avg 21.2us / max 21.7us
  ecmult_const: min 63.9us / avg 64.3us / max 64.8us
  ecmult 1: min 49.4us / avg 49.7us / max 50.3us
  ecmult 1g: min 39.8us / avg 40.0us / max 40.3us
  ecmult 2g: min 27.2us / avg 27.3us / max 27.8us
  ecmult_multi 1g: min 39.8us / avg 40.0us / max 40.2us
  ecmult_multi 2g: min 27.2us / avg 27.4us / max 27.7us
  ecmult_multi 3g: min 22.8us / avg 22.9us / max 23.1us
  ecmult_multi 4g: min 20.6us / avg 20.8us / max 21.1us
  ecmult_multi 5g: min 19.3us / avg 19.5us / max 19.7us
  ```

  (Turns out ecmult_gen is 37% of the 55.8us that schnorrsig sign takes)

ACKs for top commit:
  real-or-random:
    ACK 8f879c2887e166da2ec959ce78078f7b84ebfdf9
  elichai:
    tACK 8f879c2887e166da2ec959ce78078f7b84ebfdf9

Tree-SHA512: 8a739f5de1e2c0467c8d1c3ceeaf453b396a470ea0e8e5bef15fe1b32f3f9633b6b1c7e2ce1d94d736cf3e9adecd8f4f983ad4ba37450cd5991767f1a95db85c