smt: add support for uninterpreted ext modules

[ekiwi]

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• n/a Did you update the FIRRTL spec to include every new feature/behavior?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you state the API impact?
• Did you specify the code generation impact?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• new feature/API

API Impact

• adds a new UninterpretedModuleAnnotation which is consumed by the SMT backend to model ExtModules with uninterpreted functions / constant arrays

Backend Code Generation Impact

• uses UninterpretedFunctions in the SMTLib backend
• uses constant arrays for the btor2 backend

Desired Merge Strategy

• squash

Release Notes

The SMT backend now has support for modelling with uninterpreted functions using the UninterpretedModuleAnnotation.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels?
• Did you mark the proper milestone (1.2.x, 1.3.0, 1.4.0) ?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you mark as Please Merge?