Selectable arg selectors

[arainko]

Resolves #14

Rework the way argument selectors are handled:

• use Selectable instead of Dynamic which has better IDE supports (hints for refinements!)
• drop NamedArgument and friends in place of a simple Nothing type fallback in FunctionArguments
• workaround this dotty issue with a final class with a private constructor, of course you cannot call the contructor directly from inside an inline method but just creating a private def fixes it (AppliedViaBuilder and DefinitionViaBuilder)
• annotate all builder methods with @compileTimeOnly and add NotQuotedException instead of runtime instances
• cleanup all not private but internal classes
• change the encoding of Identity to a final class with a private constructor