nifc: add alignof & offsetof, make sizeof only take type

[metagn]

Unlike sizeof, alignof and offsetof only expect a type in C, so sizeof also now accepts only types instead of only expressions.

[Araq]

I think it's ok to change the spec so that sizeof always and only takes a type. sizeof(value) makes no sense in an optimized-for-codegens language. :-)

[metagn]

Done