branching: merge stable into default This show that the recent changes on default fixed the issue with transaction overwriting content in `test-transaction-wc-rollback-race.t`

rhg: in path_encode, simplify a bit more Use the slices for `basename` and `ext` instead of dealing with offsets.

rhg: in path_encode, be a bit more conservative about memory usage Use [shrink_to_fit] to match the previous behavior more closely, and potentially save (a tiny bit) of memory. FWIW, I suspect this is unnecessary, but this whole MR is about simplifying things while preserving any existing optimizations.

rhg: small refactor: stop using a magical constant "+ 1" Instead, directly do what the "+ 1" was supposed to achive: call hash_encode.

rhg: in path_encode, use Vec directly instead of VecDest No need to have a trivial wrapper over the type. There's nothing confusing about vec.write_bytes(...), after all.

rhg: in path_encode, split Dest into VecDest and MeasureDest Two separate types make the write semantics easier to understand because we can consider the two sinks separately. Having two independent compiled functions for size measurement and for actual encoding seems likely to improve performance, too. (and maybe we should get rid of measurement altogether) Getting rid of [Dest] also removes the ugly option rewrapping code, which is good.

rhg: use generic DestArr in hash_mangle This simplifies code a bit more, but comes with an extra memory copy in case [destlen == dest_vec.len()]. This is probably fine, but a follow-up change is removing that too.

rhg: in path_encode, make DestArr generic over its size

rhg: in path_encode add a DestArr type This is an implementation of Sink trait that writes into a fixed-size buffer on the stack, so identical to what was done before, but it makes the code of [hash_encode] easier to understand by dropping all these slice manipulations.

rhg: reduce verbosity in path_encode by using a trait for writing Hopefully this makes the code easier to read and understand and shorter overall. It also lets us later tweak the type we use as a [Sink], without having to change the encoding functions, including using two different types for size measurement and for the actual serialization.