SMT2: range_type fixes

[kroening]

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 15.21739% with 39 lines in your changes missing coverage. Please review.

Project coverage is 78.91%. Comparing base (f9a7807) to head (632093c).
Report is 105 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	18.42%	31 Missing ⚠️
src/util/std_types.cpp	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8537      +/-   ##
===========================================
+ Coverage    78.38%   78.91%   +0.53%     
===========================================
  Files         1729     1729              
  Lines       200151   198796    -1355     
  Branches     18244    18357     +113     
===========================================
+ Hits        156883   156885       +2     
+ Misses       43268    41911    -1357     

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