Add an LSpace.fork method, which replaces the current LSpace

Loosely, this is intended for situations where you want to use
LSpace.[]=, but don't want those changes to propagate to your caller
or to other blocks bound to your own LSpace.

The interaction with around_filters is a bit messy, though fairly easy
to describe. I've written a test case to capture the current behavior,
though I don't actually know whether or not it's desirable, but I also
don't know what you'd do instead.