layers/GC: consider using buffered writes to the suffix file (eg ocaml out_channel, or our own version)

[tomjridge]

Currently the layers code handles append by issuing a pwrite. It may well be that using a buffered output (eg ocaml's out_channel, or our own version) would be much faster, at least in certain circumstances.

This issue arose because layers/GC snapshot import is still a bit slower than main branch snapshot import, and the likely cause is that layers issues direct syscalls whereas it could instead use buffered output.

[Ngoguey42]

Buffer writes are included in the gc branch, from the apprend only module