By moving `Certificate` to `LibCrypto` it is possible to reuse a bunch
of code from in `LibCrypto` itself. It also moves some constants
and pieces of code to a more appropriate place than `LibTLS`.
This also makes future work on WebCryptoAPI easier.
This allows the decoder to fail gracefully when reading a partial or
malformed TBSCertificate. We also now ensure that the certificate data
is valid before making a copy of it.
