Give Registry a unicode tree output

[Julian]

This is useful for debugging what's in a registry -- show some unicode-box-drawing output which breaks down all the resources in the registry.

Probably this should print URLs in some sort of tree, e.g. (without the box drawing):

http://example.com/  – Resource(...)
  foo/ – Resource(...)
    bar/ – Resource(...)
    baz/ – Resource(...)
http://example.org/ – Resource(...)

[ykd007]

Taking a crack at this — adding a display_as_tree() method to Registry that renders URIs in a unicode box-drawing tree grouped by prefix hierarchy.

[ykd007]

Hi @Julian — I submitted #355 for this but it was closed without feedback. Happy to resubmit with any adjustments needed. A few questions to make sure I get it right:

1. Any preference on the method name? (display_as_tree, pprint, __str__, etc.)
2. Should it go on Registry directly or as a standalone helper/repr?
3. Any preference on output format — e.g. printing to stdout vs returning a string?

Thanks!