fix(FileStub#readdir): Remove extra leading slash

Ex. readdir entry '/root' was stubbed as '//root'.
feat(FileStub#stubFile): Detect empty dir names

Allow trailing slashes to indicate an empty dir.
fix(FileStub): Omit trailing slash from stub keys

When creating/seeking stubs, drop trailing slashes.
test(FileStub#stubTree): Cover string argument

- Update usage example.
docs(api): Update stubTree comments
docs(api): Add notes about dropped trailing slashes
docs(changelog): Init 0.4.1
chore(pkg): Bump version