Closes [rails#408][]

Introduce a simple chainable `WhereClause` class inspired by [Active
Record][].

All methods (including those that integrate with [Enumerable][]) are
delegated to `WhereClause#all`, which itself delegates to `Base.find`.

By merging parameters through `.where`-chaining, this commit supports
deferred fetching:

```ruby
people = Person.where(id: 2).where(name: "david")
  # => GET /people.json?id=2&name=david

people = Person.where(id: 2).all(params: { name: "david" })
  # => GET /people.json?id=2&name=david

people = Person.all(from: "/records.json").where(id: 2)
  # => GET /records.json?id=2
```

[rails#408]: rails#408
[Active Record]: https://github.com/rails/rails/blob/main/activerecord/lib/active_record/relation/where_clause.rb
[Enumerable]: https://ruby-doc.org/3.4.1/Enumerable.html