Access modifier to restrict use only in the current partial declaration

[alrz]

This would be much similar to fileprivate access modifier in Swift, but instead of the "file", they would be only accessible to the current partial declaration.

partial class C {
  int Method() { .. }
}

// Generated
partial class C {
  hidden int Method() { .. } // does not conflict with `Method` in user-code
} 

Beyond code generation, it's also useful to have a more restrictive member access modifier for types that span multiple files.

Sometimes it doesn't help to make a new class for members that you want to separate out from the rest of the declaration. Using private members still doesn't prohibit use in other parts and therefore may break the separation you intended to enforce.

[Joe4evr]

Duplicate of #1288