Make nClass interfaces (R6 classes) use the same scoping for compiled as uncompiled

[perrydv]

This PR sets the parent_env of the R6 class generated to interface to a compiled nClass object to achieve the same scoping as the uncompiled nClass. There is one environment layer in between that provides the chance to place objects that will be found before scoping reaches to the uncompiled nClass parent_env when looking for a name. This change means that if an Rpublic function (including initialize) relies on scoping to find a non-member object, that will work in the compiled interface class as well. For example, this is relevant if an nClass is created in a function and uses local variables in that function, to be found by lexical scoping.

This has no effect and is much less relevant for package development anyway. In that case, typically, both the nClass and its compiler R6 interface would have parent_env that is the package namespace. In the writePackage scheme, the R6 interface class code goes into the package source code and then does not have its parent env manipulated.