Add allocation benchmarks on StaticArrays

[gdalle]

DI extensions

• ForwardDiff: Don't reuse preallocated vector of Duals in pushforward for out-of-place functions

Warning

This would cause loss of performance in other cases. The right solution would be to distinguish based on whether or not x allows setindex, but I don't want ArrayInterface.jl as a dependency.

DI tests

• ForwardDiff: Start testing absence of allocations on static scenarios

[gdalle]

Closing because outdated, follow up in #616