HPACKHeaders.peekPseudoHeader allocates awkward temporary array

[Lukasa]

To peek the pseudo header we need to allocate an array for the subscript. We ideally wouldn't do that: we want a single value from the backing storage. Probably this requires some different API on HPACKHeaders to iterate the headers rather than create the intermediate array, and then a slightly awkward additional data-structure to handle getting only the first element and checking whether the second exists.

Before fixing this we should add an allocation counter check to validate that the allocations occur and to confirm their absence.