move bitvector-related expression classes into separate header file

[kroening]

std_expr.h is by far the largest header file, which impedes on readability.
This commit moves the bitvector-related expression classes into a separate
header file, following the pattern in mathematical_expr.h.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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/
• n/a 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

Merging #5759 (7b0cc28) into develop (0f57d77) will decrease coverage by 0.00%.
The diff coverage is 94.39%.

@@             Coverage Diff             @@
##           develop    #5759      +/-   ##
===========================================
- Coverage    69.65%   69.65%   -0.01%     
===========================================
  Files         1243     1245       +2     
  Lines       100843   100842       -1     
===========================================
- Hits         70243    70242       -1     
  Misses       30600    30600              

Flag	Coverage Δ
cproversmt2	43.39% <91.26%> (+<0.01%)	⬆️
regression	66.56% <94.28%> (-0.01%)	⬇️
unit	32.20% <55.14%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../src/java_bytecode/character_refine_preprocess.cpp	55.01% <ø> (ø)
...c/java_bytecode/java_string_library_preprocess.cpp	96.86% <ø> (ø)
src/analyses/goto_check.cpp	88.35% <ø> (ø)
src/analyses/goto_rw.cpp	55.38% <ø> (ø)
src/ansi-c/c_typecheck_base.h	100.00% <ø> (ø)
src/ansi-c/expr2c_class.h	100.00% <ø> (ø)
src/goto-programs/interpreter_evaluate.cpp	31.98% <ø> (-0.13%)	⬇️
src/solvers/flattening/boolbv.cpp	69.09% <ø> (ø)
src/solvers/flattening/boolbv.h	78.57% <ø> (ø)
src/solvers/flattening/boolbv_bitwise.cpp	90.32% <ø> (ø)
... and 26 more

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[martin-cs]

If we are going the route of splitting up std_expr.h, then, yes, clearly.

[tautschnig]

I believe this is a great first step to make things more manageable for humans. Now I'm wondering whether we can also do some work to help the compiler having to do a little less work, see my comment below.

[tautschnig]

Copy&paste error.

[kroening]

Fixed.

[tautschnig]

I understand that we need this right now, but could we consider adding something like abstract_expr.h that declares the likes of unary_exprt so that we can eventually speed up compile time a bit?

[kroening]

Yes, that's the plan. I am considering to have unary_exprt, binary_exprt etc. in expr.h.