The compiler now provides a server protocol for an interactive session
with another process. The build runner uses this protocol to communicate
compilation errors semantically from zig compiler subprocesses to the
build runner.
The protocol is exposed via stdin/stdout, or on a network socket,
depending on whether the CLI flag `--listen=-` or e.g.
`--listen=127.0.0.1:1337` is used.
Additionally:
* add the zig version string to the build runner cache prefix
* remove --prominent-compile-errors CLI flag because it no longer does
anything. Compilation errors are now unconditionally displayed at the
bottom of the build summary output when using the terminal-based
build runner.
* Remove the color field from std.Build. The build steps are no longer
supposed to interact with stderr directly. Instead they communicate
semantically back to the build runner, which has its own logic about
TTY configuration.
* Use the cleanExit() pattern in the build runner.
* Build steps can now use error.MakeFailed when they have already
properly reported an error, or they can fail with any other error
code in which case the build runner will create a simple message
based on this error code.
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