Breaking Changes

• ManagedFile has had its metadata functions moved to a new trait File which
  ManagedFile must be an implementor of. This allows dyn File to be used
  internally. As a result, PagedWriter no longer takes a file type generic
  parameter.
• ManagedFile has had its functions open_for_read and open_for_append have
  been moved to a new trait, ManagedFileOpener.
• FileManager::replace_with now takes the replacement file itself instead of
  the file's Path.
• compare_and_swap has had the old parameter loosened to &[u8], avoiding
  an extra allocation.
• TreeFile::push() has been renamed TreeFile::set() and now accepts any type
  that can convert to `ArcBytes<'static>.

Added

• AnyFileManager has been added to make it easy to select between memory or
  standard files at runtime.
• Tree::first[_key](), TransactionTree::first[_key](), and
  TreeFile::first[_key]() have been added, pairing the functionality provided
  by last() and last_key().