secp256k1_fe_sqrt checks for success

- secp256k1_fe_sqrt now checks that the value it calculated is actually a square root.
- Add return values to secp256k1_fe_sqrt and secp256k1_ge_set_xo.
- Callers of secp256k1_ge_set_xo can use return value instead of explicit validity checks
- Add random value tests for secp256k1_fe_sqrt
Merge pull request #19

09ca4f3 secp256k1_fe_sqrt checks for success (Peter Dettman)