An i2d compatibility function is rather long, so add CBB_finish_i2d for
part of it. It takes a CBB as input so only a 'marshal' function is
needed, rather than a 'to_bytes' one.
Also replace the *inp d2i update pattern with a slightly shorter one.
Change-Id: Ibb41059c9532f6a8ce33460890cc1afe26adc97c
Reviewed-on: https://boringssl-review.googlesource.com/6868
Reviewed-by: Adam Langley <agl@google.com>
