Merge pull request #75

a9f5c8b Introduce secp256k1_scalar_t for future constant-time mod order operations (Pieter Wuille)
Switch scalar to use get/set 32-byte arrays
Add unit tests for scalars.

Also add a secp256k1_scalar_is_one function.
Const scalar implementation.

This is a secp256k1_scalar_t implementation using 8 32-bit limbs (very portable,
but suboptimal on 64-bit architectures).
Get rid of secp256k1_scalar_init/free.