RFC: remove restrictions from classes/interfaces/traits

Mostly inherited from PHP5, for example:
- can't put protected methods in interfaces
- can't do `instanceof MyTrait`

There are also restrictions we've added, eg interfaces can't require implements (#7353)[

While this is technically a BC break, it's only one in that it makes HHVM more permissive - i.e. it will make previously-invalid code valid. Precedent:
- we allow covariant return types and contravariant parameter types, PHP does not
- all the PHP7 features that are enabled in PHP5 mode