make dyno::non_owning_storage the default

[jwaterloo]

dyno::non_owning_storage should be the default for the following reasons
additional heap allocations are not being encourage which goes contrary to the manifesto of the dyno library
It matches the value based semantics expected from those working with function pointers.
It makes sense for all the reasons being proposed in the function_ref proposal.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0792r0.html
function_ref is the missing link and logical development of C function pointers
dyno just increases the arity of function to greater than 1

[ldionne]

dyno::non_owning_storage is actually a special case which does not have value semantics. For this reason, I think it does not make sense to make it the default.