Collapse ssl3_state_st (s3) into ssl_st

With the removal of SSLv2, the s3 structure is always allocated, so
there is little point in having it be an allocated pointer. Collapse
the ssl3_state_st structure into ssl_st and fixup any references.

This should be faster than going through an indirection and due to
fewer allocations, but I'm not seeing any significant performance
improvement; it seems to be within the margin of error in timing.

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7888)