Enable multiple dispatch for compact layers

[MilesCranmer]

This is just an idea to make CompactLayer have a type parameter S::Symbol which lets you use multiple dispatch on the output of @compact. This addresses @darsnack's point in #12 (reply in thread).

Here's an example:

model1 = @compact(W=randn(32)) do x
  W .* x
end

model2 = @compact(MyCustomLayer, W=randn(32)) do x
  W .* x
end

my_custom_function(::CompactLayer) = 1
my_custom_function(::CompactLayer{:MyCustomLayer}) = 2

my_custom_function(model1) # 1
my_custom_function(model2) # 2

It also prints the symbol as you would expect:

julia> model2 = @compact(MyCustomLayer, W=randn(32)) do x
         W .* x
       end
@compact(
  MyCustomLayer,
  W = randn(32),                        # 32 parameters
) do x 
    W .* x
end