Add CBB_add_asn1_uint64.

Companion to CBS_get_asn1_uint64. Also add tests for both the parsing and the
serializing.

Change-Id: Ic5e9a0089c88b300f874712d0e9964cb35a8c40b
Reviewed-on: https://boringssl-review.googlesource.com/1999
Reviewed-by: Adam Langley <agl@google.com>