goto_rw/range_spect: wrap in a class instead of using just a typedef [blocks: #6749, #6768]

[tautschnig]

This helps avoiding magic "-1" constants in this code (which now is
range_spect::unknown()).

• 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]

This PR requires #6723 to build.

[codecov]

Codecov Report

Merging #6753 (a786ee0) into develop (3df1868) will increase coverage by 0.73%.
The diff coverage is 88.94%.

@@             Coverage Diff             @@
##           develop    #6753      +/-   ##
===========================================
+ Coverage    77.04%   77.78%   +0.73%     
===========================================
  Files         1594     1567      -27     
  Lines       185287   179696    -5591     
===========================================
- Hits        142763   139782    -2981     
+ Misses       42524    39914    -2610     

Impacted Files	Coverage Δ
jbmc/src/jbmc/jbmc_parse_options.cpp	76.51% <ø> (+0.53%)	⬆️
src/cbmc/cbmc_parse_options.cpp	79.90% <ø> (+0.87%)	⬆️
.../goto-instrument/accelerate/acceleration_utils.cpp	2.22% <ø> (ø)
src/goto-instrument/cover_basic_blocks.h	100.00% <ø> (ø)
src/goto-instrument/source_lines.h	100.00% <ø> (ø)
src/goto-instrument/unwind.h	100.00% <ø> (ø)
src/goto-programs/allocate_objects.h	100.00% <ø> (ø)
src/goto-programs/builtin_functions.cpp	58.35% <0.00%> (ø)
src/goto-programs/goto_check.cpp	100.00% <ø> (+18.60%)	⬆️
src/goto-symex/symex_main.cpp	86.97% <ø> (-0.04%)	⬇️
... and 86 more

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[peterschrammel]

🎉