remove typet::subtype()

[kroening]

This adds specialised variants of typet::subtype() to the three inheriting classes that use the method. These cannot use type_with_subtypet since C++ doesn't have mixins. The plan is to remove these methods in favor of the existing type-specific accessor methods.

After that, typet::subtype() can be removed.

• 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

Base: 78.43% // Head: 78.49% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (907e28f) compared to base (d7099ae).
Patch coverage: 71.42% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7561      +/-   ##
===========================================
+ Coverage    78.43%   78.49%   +0.06%     
===========================================
  Files         1670     1670              
  Lines       191671   191678       +7     
===========================================
+ Hits        150331   150464     +133     
+ Misses       41340    41214     -126     

Impacted Files	Coverage Δ
src/cpp/cpp_template_type.h	39.13% <0.00%> (-13.82%)	⬇️
src/util/type.h	100.00% <ø> (+9.21%)	⬆️
src/util/c_types.h	100.00% <100.00%> (ø)
src/util/pointer_expr.h	96.75% <100.00%> (+0.02%)	⬆️
src/util/bitvector_expr.h	97.41% <0.00%> (+0.46%)	⬆️
src/util/symbol_table.cpp	91.30% <0.00%> (+2.17%)	⬆️
src/util/validate_helpers.h	100.00% <0.00%> (+11.11%)	⬆️
src/big-int/bigint.cc	89.00% <0.00%> (+14.26%)	⬆️
... and 3 more

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[thomasspriggs]

I think the missing call to bitvector_typet::check should ideally be fixed before this is merged.

You mention that a lack of mixins prevents usage of type_with_subtypet, which presumably results in the need to write duplicate definitions of the subtype() member functions. I think the Curiously Recurring Template Pattern (CRTP) could be used add in subtype() member functions to the classes in the hierarchy which still require them instead. However that would be a different refactor which I do not think should hold up this PR.

[thomasspriggs]

This function should include a call to bitvector_typet::check as well, because the new function hides the existing one.