Made cpp2util.h functions constexpr/inline

For functions (not objects):

- If it was declared both `constexpr` and `inline`, removed `inline` (redundant)
- Otherwise, if it was not declared `constexpr` and all compilers agree it could be, added `constexpr`
- Otherwise, if it was a nonmember function that was not declared `inline`, added `inline`

Resolves thread starting here: 963f06f#commitcomment-143114319

Thanks @bluetarpmedia and @JohelEGP !

Author: hsutter