Add SegmentStore for per-segment async local storage
Part of a series of refactors to prepare for encoding params as Server
References. No outward behavioral changes yet, just refactoring.
Adds a per-request, per-segment storage mechanism: every route segment
has its own SegmentStore, keyed by its LoaderTree node — conceptually,
as if every route module exported its own store. The stores hang off
the WorkUnitStore, so the state they hold is request-scoped.
This will primarily be for storing objects related to params. They need
to be accessible via the module system so that React can map a Server
Reference back to its corresponding segment (eventually, once we land
this feature).
The first thing we migrate to being tracked on the SegmentStore is the
vary params accumulator. In subsequent steps, we'll move search params
and params, too.
