This avoids depending the FIPS module on crypto/bytestring and moves
ECDSA_SIG_{new,free} into the module.

Change-Id: I7b45ef07f1140873a0da300501141b6ae272a5d9
Reviewed-on: https://boringssl-review.googlesource.com/15984
Reviewed-by: Adam Langley <agl@google.com>