Due to the removal of `S3StorageClient`, simplify the infrastructure
under `/Storage/`:
* Move and rename `LocalStorageClient` to `StorageClient`, de-virtualize
all of its methods
* Move and rename `LocalWritableFile`, `LocalReadableFile` to
`WritableFile` and `ReadableFile`, de-vitualize all methods
* Drop the `Std{in,out}StorageClient`, make `FilePath` handle the
stdin/stdout logic
* Remove `setStorageCredentials` from `PipelineManager`
Giacomo Vercesi
8c76e7d2ef