Include functional for win builds

[cannc4]

I was hitting some errors due to std::functional not being available on Windows.
Specifically this setup:

-- Building for: Visual Studio 17 2022
-- The C compiler identification is MSVC 19.35.3221
-- The CXX compiler identification is MSVC 19.35.32217.1

Seems to be able to build after the includes

[nick-thompson]

Thanks @cannc4! I'm going to take this into the develop branch and cut it with the next patch release

[nick-thompson]

Hm changing the base pulled in some other commits, so instead I picked your change onto develop here: a7cd0c2. Closing here as merged, thanks again!