RFR: storage: support nested paths in file storage

[Erik Helin]

Hi all,

please review this path that makes `FileStorage` with files in nested directories.

Testing:
- `make test` passes on Linux x64
- Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - storage: support nested paths in file storage

Changes: https://git.openjdk.java.net/skara/pull/760/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=760&range=00
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/760.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/760/head:pull/760

PR: https://git.openjdk.java.net/skara/pull/760