By avoiding `import Foreign` this accomplishes backward-compatibility
w.r.t. the `unsafePerformIO` relocation w/o resorting to CPP
conditionals.

Moreover, in order to support base<4.7 this commits needs to reintroduce
a harmless backport of `newFilePath`.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>