efficient aggregates

[main--]

Need to implement toast-expanded format, alloc in the right mempool, make sure pointers are equal

• user perspective: take and return a cow-like type that's either the agg buffer or a copy
• must be first param btw
• has a method to resize
• you get the base varlena as &[u8], please build your own type (as always)

[main--]

update: so far, varlena was reworked to a DST-based approach

an intial implementation of this simply needs to hand out something like Result<&mut T, &T> for the first parameter

at some later point we could then implement extended-format varlenas as well which saves half of all allocations