Refactoring: Change `OverpassQuery` structure

[holyturt]

This branch attempts to change OverpassQuery and its related classes to avoid user to make wrong query relationships. Especially, it:

• changes the way making instance of OverpassQuery,

  
  // before
  let node = SwiftOverpass.query(type: .node)
  let way = node.related(.way)

// after let node = NodeQuery() let way = node.way()

- expresses relationships with mixins like below:
```swift
class NodeQuery: OverpassQuery, WayReferable, RelationReferable {
    // contains: way(_:), relation(_:)
}

class RelationQuery: OverpassQuery, NodeReferable, WayReferable, RelationReferable, 
 BackwardsReferable {
    // contains: node(_:), way(_:), relation(_:), backwards(_:)
}

• splits up methods which make xml query from OverpassQuery to new class XMLQueryBuilder to clarify these classes' concern and make it simpler.

This will have breaking changes and incompatibilities. Any ideas?

[wtimme]

Hey @holyturt,

Great to see your changes! I was on vacation the last couple of weeks, and just saw the pull request. I'll be going through the files and add comments as I go along, just like I would during a code review. If you have any questions, suggestions or remarks, or if you have any ideas for the process, please comment.

[holyturt]

Hey, @wtimme! Thank you for your review! That’s very helpful. I will merge this PR with confidence!