move bitvector_typet into bitvector_type.h

[kroening]

This moves the class bitvector_typet from std_types.h into bitvector_types.h, which is more intuitive.

• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.49%. Comparing base (0656298) to head (5598a32).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8962   +/-   ##
========================================
  Coverage    80.49%   80.49%           
========================================
  Files         1704     1704           
  Lines       188805   188805           
  Branches        73       73           
========================================
+ Hits        151983   151986    +3     
+ Misses       36822    36819    -3     

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