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

[angelhof]

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:

## 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.