Hi!

In certain embedded situations, you may want to avoid needless heap
allocations. Since the major compiler makers apply "small object
optimisation", it would be nice to somehow query if a type satisfies the
SOO or if std::function would do a heap allocation (something that we could
maybe static_assert in the very least if no other options are automatically
available).
 I believe that whether an object satisfies SOO or not may depend on both
size and alignment of the type, so the most straight-forward API I can
think of is something along the lines of
`std::function_is_inplace_v<FUNCTION, TYPE>` or something like that.
It would also be nice to have a "std::inplace_function" that is a stand-in
replacement for std::function when it is unfit for similar reasons. I see a
"SG14" inplace_function, but I am unsure if inplace_function has been up
for discussion before and shot down for some reason.

// Robin