Implement expression group transcription for conformance DSL.

[nirosys]

Issue #, if available: #935

Description of changes:
This PR adds support for transcribing expression groups in the toplevel clause of the DSL, similar to expression invocations.

Additionally, a small change was also made to the mactab clause to automatically include the current unnamed module when re-defining it in order to keep $ion imported.

Example usage:

(ion_1_1
   (toplevel ('#$:values' ('#$:' 0 1 2)))
   (produces 0 1 2)
)

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.58%. Comparing base (138ed13) to head (ee4a692).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #953      +/-   ##
==========================================
- Coverage   78.58%   78.58%   -0.01%     
==========================================
  Files         138      138              
  Lines       35210    35210              
  Branches    35210    35210              
==========================================
- Hits        27671    27670       -1     
  Misses       5475     5475              
- Partials     2064     2065       +1     

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