Stitching multiple codegen'd RTL files can have name collisions

Consider the case where you want to use a parameterized RTL wrapper that conditionally instantiates different XLS-generated RTL. This is OK if every module has different names, say you generate `A` and `B`. However, if both `A` and `B` spawn proc `C`, you can get multiple module definitions with the same name (it won't be `C`, it will be some mangled name, but they can still be the same).

Some possible ways to deal with this:

- Directly support generating parameterized RTL so you have one compilation unit (this seems like a big divergence from current behavior)
- Add "multiple-tops" to opt + scheduling + codegen (slightly more limited version of the above)
- Add an option that mangles extra stuff into spawned proc names
- Have a mechanism to support marking some modules as extern/already taken when generating RTL, so you'd generate RTL serially by adding all the previously taken names to a list consumed by the next compilation unit.

Any other ideas?