Port Certificate Request parsing to crypto/bytestring

Along the way, clean up the certificate types code to not have the
hard-coded fixed-size array.

Change-Id: If3e5978f7c5099478a3dfa37a0a7059072f5454a
Reviewed-on: https://boringssl-review.googlesource.com/1103
Reviewed-by: Adam Langley <agl@google.com>