`Flux.params` is broken

[mcabbott]

I know it's not really supported, but params is more broken than it should be (on master):

julia> Flux.params([2,3])
ERROR: UndefVarError: `IdSet` not defined

julia> @eval Flux using Base: IdSet

julia> Flux.params([2,3])
Params([[2, 3]])

It should probably print friendly upgrade instructions, @warn "..." maxlog=1. People calling this are probably struggling to update old code, and we should help them along.