smt2: convert mathematical_function types and function_application

[kroening]

This change enables the SMT-LIB2 backend to convert 1) function symbols with
type mathematical function and 2) applications of such functions given as
function_application expression.

This enables a number of use cases, e.g., encodings of programs into
constrained Horn clause systems.

• 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.
• 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

Merging #6709 (cace2ce) into develop (6594cbb) will decrease coverage by 0.00%.
The diff coverage is 68.51%.

@@             Coverage Diff             @@
##           develop    #6709      +/-   ##
===========================================
- Coverage    77.15%   77.14%   -0.01%     
===========================================
  Files         1582     1582              
  Lines       182628   182674      +46     
===========================================
+ Hits        140900   140929      +29     
- Misses       41728    41745      +17     

Impacted Files	Coverage Δ
src/solvers/smt2/smt2_conv.cpp	67.46% <68.51%> (-0.08%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dbc5d5d...cace2ce. Read the comment docs.