* Convert to C
* Add key generation, import/export of private keys, signing to secp256k1.h
* Unit tests for num/fieldelem/groupelem, including ones intended to
  trigger fieldelem's boundary cases.
* Build configuration
* Which selectable parts?
  * GMP/OpenSSL for num backend (add builtin version?)
  * __int128/asm_amd64/generic version for fieldelem
* Complete constant-time operations for signing/keygen