ieee_floatt::one(...)

[kroening]

This adds ieee_floatt::one(...), as a convenience helper for generating an IEEE 754 number with the value one.

• Each commit message has a non-empty body, explaining why the change was made.
• 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 78.26087% with 5 lines in your changes missing coverage. Please review.

Project coverage is 78.73%. Comparing base (6dc6dd5) to head (ebaf179).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
unit/util/ieee_float.cpp	50.00%	4 Missing ⚠️
src/goto-programs/interpreter_evaluate.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8552      +/-   ##
===========================================
- Coverage    78.93%   78.73%   -0.20%     
===========================================
  Files         1729     1730       +1     
  Lines       198762   199255     +493     
  Branches     18313    18312       -1     
===========================================
- Hits        156888   156886       -2     
- Misses       41874    42369     +495     

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