New ASN.1 embed macro.

New ASN.1 macro ASN1_EMBED. This is the same as ASN1_SIMPLE except the
structure is not allocated: it is part of the parent. That is instead of

FOO *x;

it must be:

FOO x;

This reduces memory fragmentation and make it impossible to accidentally
set a mandatory field to NULL.

This currently only works for SEQUENCE and since it is equivalent to
ASN1_SIMPLE it cannot be tagged, OPTIONAL, SET OF or SEQUENCE OF.

Reviewed-by: Rich Salz <rsalz@openssl.org>