The variable inside the if statement was unused, unliked the blocks
above and below it. Use the isa<> check instead.

Clang was up until now not able to parse nested structs or unions as a
field in the table key or leaf. Add support in the library side, as well
as python support for the exported json description.

Even after fixing the nested union issue in the preceding commits, this
example was failing. The reason was that the kernel was filling in all
bits of the bpf_tunnel_key, which in recent kernels includes non-zero
fields such as ttl. That non-zero field would break the lookup in the
tunkey2if table, which was populated assuming all extra bytes would be
zero.

Fix it by defining a simpler key structure, and copy the required fields
out from the local variable.

Fixes: #510