rename range_typet to integer_range_typet

[kroening]

1. This renames the type class range_typet to integer_range_typet to clarify that it denotes a subrange of the integers.

2. This renames the existing getters/setters to use C++ naming, as opposed to Java naming.

3. This adds the size() method, the number of integers in the range.

4. This adds the empty() method, which returns true when the range has no elements.

• 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.

[tautschnig]

(Disabling auto-merge for now as I'd like to fix a couple of inconsistencies.)

[codecov]

Codecov Report

❌ Patch coverage is 73.10924% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.53%. Comparing base (5556082) to head (ad96e40).

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	0.00%	19 Missing ⚠️
src/solvers/flattening/boolbv_typecast.cpp	0.00%	5 Missing ⚠️
src/util/arith_tools.cpp	0.00%	3 Missing ⚠️
src/util/mathematical_types.h	75.00%	3 Missing ⚠️
src/solvers/flattening/boolbv_add_sub.cpp	0.00%	1 Missing ⚠️
src/solvers/flattening/boolbv_constant.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9001      +/-   ##
===========================================
+ Coverage    80.52%   80.53%   +0.01%     
===========================================
  Files         1705     1706       +1     
  Lines       188927   188982      +55     
  Branches        73       73              
===========================================
+ Hits        152130   152204      +74     
+ Misses       36797    36778      -19     

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