dstringt::starts_with can be const

[kroening]

• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (ccec4dd) 79.10% compared to head (fc7cc8f) 79.10%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8148   +/-   ##
========================================
  Coverage    79.10%   79.10%           
========================================
  Files         1695     1695           
  Lines       196596   196596           
========================================
  Hits        155517   155517           
  Misses       41079    41079           

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