Wrong mode pushforwards and pullbacks

[gdalle]

Should we make pushforwards available in reverse mode, and pullbacks available in forward mode?
The code is already there, we only need to push it one level up in the call tree:

https://github.com/gdalle/DifferentiationInterface.jl/blob/964c85645cf232f6629f3f6cc7ebefd752c98889/src/gradient.jl#L16-L25

https://github.com/gdalle/DifferentiationInterface.jl/blob/964c85645cf232f6629f3f6cc7ebefd752c98889/src/multiderivative.jl#L46-L55

The only thing we don't have yet is a JVP in reverse mode and VJP in forward mode

[gdalle]

Closing because #64 is ugly