Special handling of arrays if they are big only

[dtolnay]

<@nox> In heapsize_derive we do https://github.com/servo/heapsize/blob/master/derive/lib.rs#L36-L42,
<@nox> could serde do something similar for array fields?
<@dtolnay> nice, yes we could
<@dtolnay> let me make a ticket
<@nox> Cool. :)
<@dtolnay> we can do a bit better even by only doing it when the array size exceeds our biggest array impl
<@nox> Yeah true.
<@nox> Don't know why we didn't actually, given we have impls too. :)