Modernize copy prevention in Emitter, YamlEmitter, and JsonEmitter (#678)
- `Emitter`, `YamlEmitter`, `JsonEmitter`: replace private/protected
undefined copy ctor declarations with explicit `= delete` in the public
section, and add the missing copy assignment `= delete`.
- Also remove the redundant undefined null-ctor declarations in
`YamlEmitter` and `JsonEmitter` — these were suppressed automatically in
C++11 by the presence of user-declared non-default constructors.