Add a rustfmt syntactic check for the Rust project

[NlightNFotis]

This PR adds a syntactic check for the Rust project using cargo fmt.
This is very similar to the checks we have for clang format, but it's
on the whole of the Rust project (given that it's so small in terms of
LOC, we can enforce strict syntax compliance to rustfmt from the start)

• 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.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Base: 78.16% // Head: 78.50% // Increases project coverage by +0.34% 🎉

Coverage data is based on head (9e84483) compared to base (9b681ad).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7512      +/-   ##
===========================================
+ Coverage    78.16%   78.50%   +0.34%     
===========================================
  Files         1663     1663              
  Lines       191297   191297              
===========================================
+ Hits        149518   150174     +656     
+ Misses       41779    41123     -656     

Impacted Files	Coverage Δ
src/util/bitvector_expr.h	97.41% <0.00%> (+0.23%)	⬆️
src/util/std_expr.h	93.15% <0.00%> (+0.26%)	⬆️
src/util/std_code.cpp	41.89% <0.00%> (+1.35%)	⬆️
src/util/expr.h	97.14% <0.00%> (+1.90%)	⬆️
src/util/dstring.h	100.00% <0.00%> (+3.22%)	⬆️
src/util/symbol_table_base.cpp	67.85% <0.00%> (+3.57%)	⬆️
src/util/source_location.cpp	94.00% <0.00%> (+6.00%)	⬆️
src/util/simplify_utils.cpp	93.67% <0.00%> (+6.75%)	⬆️
src/util/union_find.cpp	82.81% <0.00%> (+7.81%)	⬆️
src/util/type.h	98.68% <0.00%> (+7.89%)	⬆️
... and 24 more

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[esteffin]

Looks good to me