Merge #337: variable sized precomputed table for signing

Merge #337: variable sized precomputed table for signing

dcb2e3b3fff0b287d576842aabe5c79f2fe4df30 variable signing precompute table (djb)

Pull request description:

  This pull request gives an option to reduce the precomputed table size for the signing context (`ctx`) by setting `#define ECMULT_GEN_PREC_BITS [N_BITS]`.

  Motivation: Per #251 and #254, the static table can be reduced to 64kB. However, this is still too big for some of my embedded applications. Setting `#define ECMULT_GEN_PREC_BITS 2` produces a 32kB table at a tradeoff of about 75% of the signing speed. Not defining this value will default to the existing implementation of 4 bits. Statistics:

  ```
  ECMULT_GEN_PREC_BITS = 1
  Precomputed table size: 32kB
  ./bench_sign
  ecdsa_sign: min 195us / avg 200us / max 212us

  ECMULT_GEN_PREC_BITS = 2
  Precomputed table size: 32kB
  ./bench_sign
  ecdsa_sign: min 119us / avg 126us / max 134us

  ECMULT_GEN_PREC_BITS = 4 (default)
  Precomputed table size: 64kB
  ./bench_sign
  ecdsa_sign: min 83.5us / avg 89.6us / max 95.3us

  ECMULT_GEN_PREC_BITS = 8
  Precomputed table size: 512kB
  ./bench_sign
  ecdsa_sign: min 96.4us / avg 99.4us / max 104us
  ```

  Only values of 2 and 4 make sense. 8 bits causes a larger table size with no increase in speed. 1 bit runs, actually, but does not reduce table size and is slower than 2 bits.

ACKs for top commit:
  real-or-random:
    ACK dcb2e3b3fff0b287d576842aabe5c79f2fe4df30 verified that all changes to the previous ACKed 1d26b27ac90092306bfbc9cdd5123e8a5035202a were due to the rebase
  jonasnick:
    ACK dcb2e3b3fff0b287d576842aabe5c79f2fe4df30 read the code and tested various configurations with valgrind

Tree-SHA512: ed6f68ca23ffdc4b59d51525336b34b25521233537edbc74d32dfb3eafd8196419be17f01cbf10bd8d87ce745ce143085abc6034727f742163f7e5f13f26f56e