Release CBMC 6.4.0

[tautschnig]

This release improves upon automated assigns-clause inference for loop invariants, which should make manually adding assigns clauses to loops less frequent.

• 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.
• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 78.79%. Comparing base (f5f0672) to head (27a27ca).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8495   +/-   ##
========================================
  Coverage    78.79%   78.79%           
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  Files         1727     1727           
  Lines       198783   198783           
  Branches     18532    18514   -18     
========================================
  Hits        156638   156638           
  Misses       42145    42145           

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