reduce type nesting

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There are some issues in Swift 4.1 with adding constraints to the SQLQuery protocol to reduce the amount of type nesting required, for example:

public func set(_ identifier: Connection.Connection.Query.Update.Identifier, to expression: Connection.Connection.Query.Update.Expression)

Once Swift 4.2 can be a requirement, we can reduce a lot of this nesting and make things cleaner.

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This is fixed in SQLKit 3.0