Initialization; Persistent, Lazy Filters

These commits introduce initialization hooks and solve the issue of filters being created on require.

The hooks were originally created to literally create the filter when it `stack-initialize` is called, but a re-implementation of filter access (prototyped by @Bruce-Connor) rendered this unnecessary.
Pending review, we might want to change this to `sx-filter-get` and `sx-filter--get` (or `sx-filter--get-raw`).  It's pretty inconsequential, but I anticipate `sx-filter-get-var` (as it is named now) will be used much more often than `sx-filter-get`.