There's many ways to serialize a BIGNUM, so not including asn1 in the name is
confusing (and collides with BN_bn2cbb_padded). Since BN_asn12bn looks
ridiculous, match the parse/marshal naming scheme of other modules instead.

Change-Id: I53d22ae0537a98e223ed943e943c48cb0743cf51
Reviewed-on: https://boringssl-review.googlesource.com/6822
Reviewed-by: Adam Langley <alangley@gmail.com>