Don't allow variable names to contain __

[developedby]

__ is reserved for generated function names.
At first glance it might look like this is not a problem, since generated functions are only created after resolving variable names.
However, if this is no longer true, as will be the case with the package manager, users will be able to call generated functions, which shouldn't be possible.

We should move the restriction of not having "__" from the top-level name parser to the general name parser.