move: Move BoundedPool classes into separate folder

[RustanLeino]

This is a "move" PR. The only changes are to

• move classes around to separate files (in the new Resolver/BoundsDiscovery folder)
• move BoundsDiscovery.cs into the new folder
• move all BoundedPool class out of ComprehensionExpr to be top-level classes in Microsoft.Dafny
• reformat a few long source lines by inserting newlines

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[keyboardDrummer]

Thanks!