`Base.summarysize` incorrect for `Memory` containing `Union`

[PatrickHaecker]

As discussed in the corresponding Discourse topic, Base.summarysize accounts the union optimization byte twice. Copying from the discussion:

julia> sizeandsummary(x) = sizeof(x), Base.summarysize(x)

julia> 1:100 |> Memory{UInt8} |> sizeandsummary
(100, 116)

julia> 1:100 |> Memory{Union{Nothing,UInt8}} |> sizeandsummary
(200, 316)

Where the last number should be 216, not 316.

The relevant code in summarysize.jl is as follows

        dsize = sizeof(obj)
        T = eltype(obj)
        if isbitsunion(T)
            # add 1 union selector byte for each element
            dsize += length(obj)
        end

The line dsize = sizeof(obj) accounts for the union byte array (see above). As Union{Nothing, UInt8} |> Base.isbitsunion (unsurprisingly) returns true, the union byte array is accounted a second time in dsize += length(obj).