cxplat: use of inline functions and macros

[dthaler]

In #89, comments ehre:

Nick: General question though: Can/should we move stuff to be header only (via macros or possibly inline functions) instead of functions defined in separate files? Ideally the linker should do a good job of eliminating overhead, but it doesn't always do it perfect.

Dave: The downside of macros/inlines is that if they're non-trivial and used a lot, then they can increase the footprint and memory requirements. So I would say: yes for trivial functions, no for complex functions. Can do a pass in a separate PR.