Make value_and_pullback_function a primitive instead of pullback_function

[sethaxen]

Currently the internals use closures to avoid computing the primal more than once, but it would be easier to read, simpler, and more consistent with ChainRules, Zygote, Diffractor, etc to instead make value_and_pullback_function the primitive. Should come after #4.

Not certain if the same should be done for pushforward_function.