During export, check for identifier collisions in output

[amykyta3]

See example in https://github.com/SystemRDL/systemrdl-compiler/issues/193

Current implementation will silently emit colliding output, or even reuse an incorrect typedef. Monitor for this and emit an error if it occurs.

Ideas for fix:

• Temporarily generate prefixes using non-underscore characters that are impossible to specify in RDL source.
• Use the above prefix as the source of truth to uniquely identify a type name
• Replace special delimiter chars with underscores and check if it collides with an existing scope prefix elsewhere.