.FastAIwithfields

			withfields(f, x; kwargs...)

Replace fields on x with given keyword arguments, run f and then restore the fields. x needs to be a mutable struct.

Every keyword argument is a mapping (field, value) or (field, (setfn!, value)). setfn!(x, val) will be used to set the field; if as in the first case none is given setfield! is used.