Pretty-printing of multi-variable let expressions

[tautschnig]

Fix expr2c and format to support LET expressions over multiple
variables. expr2c in fact wouldn't even correctly support
single-variable LET expressions anymore as it wasn't updated in
71a20e9.

• 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/
• 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 #6516 (43f2fa7) into develop (6f7c25f) will increase coverage by 0.00%.
The diff coverage is 42.50%.

@@           Coverage Diff            @@
##           develop    #6516   +/-   ##
========================================
  Coverage    76.10%   76.11%           
========================================
  Files         1548     1548           
  Lines       166326   166312   -14     
========================================
- Hits        126587   126585    -2     
+ Misses       39739    39727   -12     

Impacted Files	Coverage Δ
src/ansi-c/expr2c.cpp	66.34% <0.00%> (+0.23%)	⬆️
src/goto-diff/goto_diff_parse_options.cpp	58.02% <0.00%> (-1.03%)	⬇️
src/goto-diff/goto_diff_parse_options.h	100.00% <ø> (ø)
.../goto-instrument/goto_instrument_parse_options.cpp	69.14% <ø> (ø)
...rc/goto-instrument/goto_instrument_parse_options.h	100.00% <ø> (ø)
src/goto-instrument/unwindset.h	100.00% <ø> (ø)
src/util/format_expr.cpp	86.71% <0.00%> (-2.80%)	⬇️
src/goto-instrument/unwindset.cpp	95.91% <93.33%> (+11.13%)	⬆️
src/analyses/goto_check_c.cpp	90.28% <100.00%> (-0.01%)	⬇️
src/util/ieee_float.cpp	88.01% <0.00%> (+0.16%)	⬆️
... and 2 more

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