As a first round, rewrite those handshake message types which can be
reused in TLS 1.3 with golang.org/x/crypto/cryptobyte. All other types
changed significantly in TLS 1.3 and will require separate
implementations. They will be ported to cryptobyte in a later CL.

The only semantic changes should be enforcing the random length on the
marshaling side, enforcing a couple more "must not be empty" on the
unmarshaling side, and checking the rest of the SNI list even if we only
take the first.

Change-Id: Idd2ced60c558fafcf02ee489195b6f3b4735fe22
Reviewed-on: https://go-review.googlesource.com/c/144115
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>