implement FilenameMap.show and pp
implement FilenameMap.show and pp

Summary:
`FilenameMap` is a `WrappedMap`, which already defines a printer, but you have to expose it by passing in a printer for the key type.

enables using `FilenameMap` with `[@deriving.show]` and `[%derive.show]`

Reviewed By: panagosg7

Differential Revision: D25561103

fbshipit-source-id: 0893a0c5ba290a77dbec68b4489c34f3c4198dd0

facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow