Previously, `zig fmt` on the stage1 compiler (which is what we currently
ship) would perform what equates to `zig run std/special/fmt_runner.zig`

Now, `zig fmt` is implemented with the hybrid zig/C++ strategy outlined
by #1964.

This means Zig no longer has to ship some of the stage2 .zig files, and
there is no longer a delay when running `zig fmt` for the first time.