Previously we were allocating memory on the message's arena every time we performed a `map[key]` or `map.get(key)` operation. This is unnecessary, as the key's data is only needed ephemerally, for the duration of the lookup, and we can therefore alias the Python object's string data instead of copying it.
This required fixing a bug in the convert.c operation. Previously in the `arena==NULL` case, if the user passes a bytes object instead of a unicode string, the code would return a pointer to a temporary Python object that had already been freed, leading to use-after-free. I fixed this by referencing the bytes object's data directly, and using utf8_range to verify the UTF-8.
Fixes: https://github.com/protocolbuffers/protobuf/issues/14571
PiperOrigin-RevId: 578563555
This change moves almost everything in the `upb/` directory up one level, so
that for example `upb/upb/generated_code_support.h` becomes just
`upb/generated_code_support.h`. The only exceptions I made to this were that I
left `upb/cmake` and `upb/BUILD` where they are, mostly because that avoids
conflict with other files and the current locations seem reasonable for now.
The `python/` directory is a little bit of a challenge because we had to merge
the existing directory there with `upb/python/`. I made `upb/python/BUILD` into
the BUILD file for the merged directory, and it effectively loads the contents
of the other BUILD file via `python/build_targets.bzl`, but I plan to clean
this up soon.
PiperOrigin-RevId: 568651768
