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Reuse ByteString/ByteBuilder APIs for CT parsing and serialization.

Added boolean return arguments to EncodeTreeHeadSignature and
EncodeSignedCertificateTimestamp, and updated a couple of clients to check
return value and handle failure cases as appropriate.

Bug: 634570
Change-Id: Id309470e18d0bc7316623c2f5dd7aaf3854527cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1465984
Commit-Queue: Kaustubha Govind <kaustubhag@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637879}