evaluate: check element key vs. set definition

evaluate: check element key vs. set definition

Included bogon asserts with:
 src/datatype.c:253: symbolic_constant_print: Assertion `expr->len / BITS_PER_BYTE <= sizeof(val)' failed.

Resolve this by validating that the set element key matches the set key
definition.

After this, loading the bogon file gives:
Error: Element mismatches set definition, expected concatenation of (IPv4 address, integer), not 'ICMP type'
elements = {redirect }
           ^^^^^^^^

Signed-off-by: Florian Westphal <fw@strlen.de>