`parallel` is used in exactly one place in the GHC tree: the T2317 test.
It seems almost by accident that it is a submodule; libraries needed
only for tests should net be included as submodules (see `QuickCheck`,
`async`, `haskell98`, `regex-compat`, `utf8-string`, `vector` and more
for examples).

T2317 will now get run only when `parallel` is installed instead of
`parallel` being required for the testsuite to run.