Add `find` to method to `ReadModel`

This adds a `find` method to `ReadModel` that functions similar
to ActiveRecord's find. It takes an id and will return a single
record matching that id, if possible. It uses the same relation
building logic as `query`. The only difference is it ignores any
limit and offset expressions and removes any ordering that was
added.

There are a few notes to be aware of:

* Since `where` and `order` are `merge` expressions