User namable anonymous modules

[yav]

Some constructs in the module system introduce implicitly defined modules/interfaces. At the moment these have pretty long names, which are not valid Cryptol identifiers. On the one hand, this is good because it avoids collisions with user-defined names.

On the other hand, this is a problem for other tools integrating with Cryptol: for example if we want to reason about a Cryptol specification, we may want to refer to things defined in these implicit modules, but we can't do it, because we have no convenient way to name the anonymous modules.

To solve this we should introduce a different naming scheme for implicit modules introduce by the module system.