evaluate: prepare to store expr key rather than datatype

evaluate: prepare to store expr key rather than datatype

currently set definitions store a datatype rather than
an expression.

In order to support use of unqualified data types (string in particular),
this prepares implicit set definition helper to expect an expression instead
of plain data type.  This also has the advantage that we can use EXPR_CONCAT
to retain the original expressions when key concatentation is used, e.g.
'meta iifname . tcp dport'.  The netlink serialization code can use
this info to store individual key lengths independently of data types.

Would also allow later on to store the original names of the
expressions, e.g. "ip daddr", in the kernel to support a future
typeof keyword, e.g. 'type typeof(ip daddr)' instead of 'type ipv4_addr'.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>