No longer emit accessors for private fields

[lrytz]

Except for value class and case class parameters.

For value class parameters, the accessor is used as the unbox method in
other classes. There are a few assumptions in the compiler for that method
being present.

Private case class parameters are excluded because an accessor is generated
anyway to support extraction. We could change the backend avoid going through
the accessor within the class.

Mark object fields static in CleanUp, because:

class C { def f = C.x }
object C { private val x = 0 }

The field x is made notPrivate to allow access from C. In the
backend, the field is emitted static. However, if only the backend
phase adds the static flag to the symbol, it's not there yet when
emitting C, and therefore the field access has the wrong opcode.

[lrytz]

Linting for unused privates would need to be fixed, as now there are no more unused accessors, only the field which is accessed in the constructor - should be doable, as linting works correctly for private[this].

[retronym]

I can imagine that this could break some reflection based frameworks that are looking for annotated getter methods that we no longer emit. So maybe we need to expose this as a compiler setting. One of the options could be to emit them for annotated fields.