Add static_assert for pInfo field widths.

[rillian]

Check at build time that the fixed-length fields in the struct we call memcpy against are actually the size we expect. This should help detect buffer overflow if the p11 library header changes under us.

[JackOfMostTrades]

I have a hard time imagining the ABI for pkcs11 modules would change since that would probably break all sorts of stuff, but it's a good sanity check to have at any rate. :)