The purpose of this small (in scope and footprint) module is simple: spawn a
subprocess asynchronously as efficiently and/or fast as possible. Basically the
same as calling fork+exec (on POSIX), but hopefully faster than those two
syscalls.

Apart from fork overhead, this module also allows you to fork+exec programs when
otherwise you couldn't - for example, when you use POSIX threads in your perl
process then it generally isn't safe to call fork from perl, but it is safe to
use this module to execute external processes.