From https://github.com/Naios/function2/



This provides unique_function, a move-only, type-erased function
wrapper.

I copied the file in rather than using a submodule because it's only
one file, and a submodule seems rather like killing a mosquito with an
anti-tank gun.

Chosen over the slightly more capable cxx_function because that
version fails its basic contract.

Signed-off-by: Adam C. Emerson <[email protected]>