Give FileSystemHandle an associated path

A much more thorough (than #30) attempt at expressing that FileSystemHandle maps to a file path. At a high level, it proposes that:

* A FileSystemHandle has an associated "file system locator"
* A "file system locator" contains, among other things, a "file system path"
* A "file system entry" can be located given a "file system locator"
* You can get a "file system locator" from a "file system entry"

See #59 for the implications of this in practice.

Fixes #59.