After https://codeberg.org/ziglang/zig/pulls/30103, `raw_c_allocator` is
redundant. It existed to avoid overhead when you could assert that all
of your `Allocator` usage was going to be compatible with the C `malloc`
API, but the standard `c_allocator` is now able to avoid that overhead
*in the case* that your usage is compatible (and use the less efficient
path in the rare case where it's not), so there's no need for the raw
version anymore. Leaving it in `std.heap` at this point seems like it
would just be a footgun.