Move the TLSv1.3 handshake struct inside the shared handshake struct.

Move the TLSv1.3 handshake struct inside the shared handshake struct.

There are currently three different handshake structs that are in use -
the SSL_HANDSHAKE struct (as S3I(s)->hs), the SSL_HANDSHAKE_TLS13 struct
(as S3I(s)->hs_tls13 or ctx->hs in the TLSv1.3 code) and the infamous
'tmp' embedded in SSL3_STATE_INTERNAL (as S3I(s)->tmp)).

This is the first step towards cleaning up the handshake structs so that
shared data is in the SSL_HANDSHAKE struct, with sub-structs for TLSv1.2
and TLSv1.3 specific information. Place SSL_HANDSHAKE_TLS13 inside
SSL_HANDSHAKE and change ctx->hs to refer to the SSL_HANDSHAKE struct
instead of the SSL_HANDSHAKE_TLS13 struct. This allows the TLSv1.3 code
to access the shared handshake data without needing the SSL struct.

ok inoguchi@ tb@