Minimal support for foward mode

[mcabbott]

This just adds empty stubs for pushforward functions. This should be enough to let you call them within a backward-mode rule @adjoint map(f, xs) = ....

It is not enough to let you define new forward rules. I'm not sure whether @tangent is stable enough to also move over... that's a bigger job which could be done later.

[mcabbott]

It may make sense to similarly define forwarddiff, extract, seed, which would allow the use of ForwardDiff within rules, without depending on that package. (As proposed in #9, too).