Move extension methods into the extended type

[jonsequitur]

    // Guidline: DO NOT put extension methods in the same namespace as the extended type unless the intention is to add methods to interfaces, for generic type restriction, or for dependency management.
    //
    // The extension methods on TArgument should all move to be instance methods on the Argument class.  Since they were implemented as extension methods, I presume they would be non-virtual.
    // The ExistingOnly extension methods are "generic type restriction", so they're OK.