Implement a system built on 'queries' for selecting appropriate relays.
A query is a set of constraints which dictates which relay(s) that *can*
be chosen by the relay selector.
The user's settings can naturally be expressed as a query. The semantics
of merging two queries in a way that always prefer user settings is
defined by the new `Intersection` trait.
Split `mullvad-relay-selector` into several modules:
- `query.rs`: Definition of a query on different types of relays. This
module is integral to the new API of `mullvad-relay-selector`
- `matcher.rs`: Logic for filtering out candidate relays based on a
query.
- `detailer.rs`: Logic for deriving connection details for the selected
relay.
- `tests/`: Integration tests for the new relay selector. These tests
only use the public APIs of `RelaySelector` and make sure that the
output matches the expected output in different scenarios.
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