When upgrading to the new std lib HashMap API, the process_headers code
regressed because something that was supposed to be a pointer ended up
being a copy of a value. This resulted in the modification of a field
not being picked up.
Also switch from Sha256 to Blake3 while we're at it.
Instead of having all primitives and constructions share the same namespace,
they are now organized by category and function family.
Types within the same category are expected to share the exact same API.
This is the beginning of supporting minimum GLIBC version as part of the
target. See #2509 for the motivation.
The dummy libc zig files are removed. A future commit will build them
on-the-fly, using the generated text files generated by the new tool,
which are checked into source control and distributed along with zig.
These generated text files are, together, 142KB (20KB gzipped).
Compare that to a naive bundling of the .abilist files, which would be
2.2MiB (375KB gzipped).
This is based on glibc 2.29.
