Parallelize the call to the compiler with the execution of the command

We could improve pash by at the same time calling the compiler and start to execute the original command.

This would give some benefits (if done fast---which seems very hard) for scripts where command execution is so small that it ends even before the compiler finished its invocation.

Below follows a solution schematic from an early prototype:
```python
## Solution Schematic:
##
## (A) (B) (C) (D) (E)
## stdin --- tee --- eager --- seq.sh --- eager --- OUT_SEQ
## \ (F)
## \--- eager --- PAR_IN
##
## (1) If compiler fails, or sequential is done executing:
## - cat OUT_SEQ > stdout
##
## (2) If compiler succeeds:
## - USR1 to reroute so that it redirects to /dev/null
## - PAR_IN redirect to par stdin.
```

No implementation files, tests, or entry points are named. Start by locating the compiler invocation and the execution path for the original command, then compare them with the schematic in the issue. Done means both can proceed concurrently, compiler failure or early sequential completion preserves stdout, and compiler success reroutes input as described.

Written by the indexing model from the issue text.