Don't generate code that can access Dafny runtime internals

[robin-aws]

It is currently possible when compiling to .NET to reference symbols that are intended to be internal to the runtime, such as Dafny.ArraySequence. I believe this is because the generated code is placed in the same namespace as the runtime, at least when embedding the runtime source into the generated *.cs file. Or perhaps it is because the code is all in the same compilation unit (i.e. file) - I admit I am relatively new to .NET so I'm not 100% sure.

In any case, it must be possible to hide details of the runtime from user code, even if marked with {:extern}, and the particulars of how the code is generated should not affect the access control.