Factor out assign_metadata! from @components

[cstjean]

@components is used a lot, and its macroexpansion contains code that will be compiled over and over again. This PR pulls that code into a helper function, such that it will be compiled only once. It should help (ever so slightly) with TTFX and compiled code size.

The macroexpansion is also cleaner:

julia> @macroexpand1(@component function decay(; name)
           @parameters a
           @variables x(t) f(t)
           System([
                   D(x) ~ -a * x + f
               ], t;
               name = name)
       end) |> striplines
quote
    (Base).@__doc__ function decay(; name)
            function var"##create_component#3103"()
                begin
                    @parameters a
                    @variables x(t) f(t)
                    System([D(x) ~ -a * x + f], t; name = name)
                end
            end
            return (ModelingToolkitBase.assign_metadata!)(var"##create_component#3103"(), false, :decay, @__MODULE__())
        end
end

vs

quote
    (Base).@__doc__ function decay(; name)
            res = ((()->begin
                        begin
                            @parameters a
                            @variables x(t) f(t)
                            System([D(x) ~ -a * x + f], t; name = name)
                        end
                    end))()
            if (isdefined)(res, :gui_metadata) && (getfield)(res, :gui_metadata) === nothing
                name = :decay
                if false
                    (Setfield).@set! res.connector_type = (ModelingToolkitBase.connector_type)(res)
                end
                (Setfield).@set! res.gui_metadata = (ModelingToolkitBase.GUIMetadata)((GlobalRef)(@__MODULE__(), name))
            else
                res
            end
        end
end