remove deprecated pool logic and replaced with async

[pollend]

The current 0.16 branch of zig has deprecated Pool in favor of Io.Thread and Io.Group . This just updates all the threading logic with this change.

https://codeberg.org/ziglang/zig/commit/985a3565c6130c7279319e9c36642f0b958e6944#diff-f192d18f39c389fafe62a3a341b75c760701fcae

[Techatrix]

Thanks for the PR. It looks like the Io interface is being used in way that doesn't quite match what I would have expected. Here are some examples:

• Most of the code is directly using a specific Io implementation like std.Io.Threaded instead of using the std.Io interface.
• There are multiple instances of std.Io.Threaded being used. The main function in main.zig, build_runner.zig and analysis_check.zig already created one at the start.
• Do not discard the return value of std.Io.async, it must be awaited or cancelled
• Use std.testing.io to get an io instance in tests

I can save you the effort of addressing these issues since I've already implemented this is #2537.