Document design decisions

[hermanventer]

Description

Document design decisions and update tool chain. Update main.rs to initialize rustc logging if the RUSTC_LOG environmental variable is set. Treat mir::NullOp::Box as unreachable since the rust compiler no longer generates it
and it is about to be removed.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• API change with a documentation update
• Additional test coverage
• Code cleanup or just keeping up with the latest Rustc nightly

How Has This Been Tested?

./validate.sh

[codecov]

Codecov Report

Merging #1122 (5f2aa56) into main (1aa4703) will decrease coverage by 0%.
The diff coverage is 0%.

@@          Coverage Diff          @@
##            main   #1122   +/-   ##
=====================================
- Coverage     77%     77%   -1%     
=====================================
  Files         23      23           
  Lines      16090   16087    -3     
=====================================
- Hits       12505   12494   -11     
- Misses      3585    3593    +8     

Impacted Files	Coverage Δ
checker/src/block_visitor.rs	78% <0%> (+<1%)	⬆️
checker/src/abstract_value.rs	78% <0%> (-1%)	⬇️

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