Add offsetof functionality

[Jopyth]

It does not belong into any of the existing folders, any (naming) suggestions on the category of this kind of functionality?

[sbusch42]

What about 'memory'? Could get other members such as make_unique etc. (if needed)

[scheibel]

I like the idea of memory although I wonder where to discriminate between functionality like make_unique and contains (e.g., on strings or containers).
Basically, I wish for another module where STL extensions can be located. make_unique would be such an STL extension, contains would also be. However, offsetof is more or less a language or compiler extension, not relying on the STL at all.

[scheibel]

I also want to add the rationale for this functionality:

There is an inofficial offsetof language extension that is mainly implemented as compiler extension but it is rather limited in usage. Lately, some compilers even dropped support of this functionality.
However, I think we understand the limitations of the implementation and won't use it in problematic cases.

[scheibel]

This PR is part of #6.

[scheibel]

ANother empty line here, please.