dillon1000/react

Use distinct type for scope dependencies

This is a pre-req to deleting the `Place.memberPath` field. We no longer need 
memberPath in the HIR now that we have PropertyStore and PropertyLoad. However, 
scope dependencies use memberPath to track the precise fields that a computation 
depends upon. This PR changes scopes dependencies to use a new 
`ReactiveScopeDependency` type (Place + optional path), which allows the next PR 
to remove `Place.memberPath`.