First parallel-compiles release behaved badly on compile errors; this is basically Parallel Compiles Take Two. I also almost completely rewrote the src/lib/std/src/posix/spawn–premicrothread.api api per suggestion from Joe Wells.
As before, if parallel compiles act up, they can be disabled by setting compile_in_subprocesses to FALSE in src/lib/compiler/toplevel/main/compiler-controls.pkg and recompiling, or doing
eval: set_control "compiler::compile_in_subprocesses" "FALSE";
at the Mythryl interactive prompt.