Split ieee_floatt into ieee_float_valuet and ieee_floatt

[kroening]

This splits ieee_floatt into two parts:

1. ieee_float_valuet stores an IEEE 754 floating-point value. It offers no operations that require rounding, and hence, does not require a rounding mode.

2. ieee_floatt extends ieee_float_valuet with a rounding mode, and hence, offers operators that require rounding.

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• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
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[codecov]

Codecov Report

Attention: Patch coverage is 65.74074% with 111 lines in your changes missing coverage. Please review.

Project coverage is 77.95%. Comparing base (c66ffbe) to head (d73f9ab).
Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/util/ieee_float.cpp	53.96%	93 Missing ⚠️
src/ansi-c/goto-conversion/goto_check_c.cpp	40.00%	6 Missing ⚠️
src/goto-programs/interpreter_evaluate.cpp	0.00%	4 Missing ⚠️
src/util/ieee_float.h	91.42%	3 Missing ⚠️
src/analyses/interval_domain.cpp	0.00%	2 Missing ⚠️
...src/java_bytecode/java_bytecode_convert_method.cpp	75.00%	1 Missing ⚠️
src/goto-programs/interpreter.cpp	0.00%	1 Missing ⚠️
src/util/simplify_expr.cpp	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8550      +/-   ##
===========================================
- Coverage    78.85%   77.95%   -0.91%     
===========================================
  Files         1732     1732              
  Lines       199119   201434    +2315     
  Branches     18560    18549      -11     
===========================================
+ Hits        157023   157035      +12     
- Misses       42096    44399    +2303     

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[tautschnig]

I very much like the idea, but I'm a bit confused: several places (some of which I've commented on below) appear to newly be drawing a rounding mode out of thin air when I'd naively have expected for those to be ieee_float_valuet. Can you please clarify?