use std::string_view in dstring

[kroening]

This introduces means to construct dstringt from std::string_view, in the hope of avoiding copies of string contents.

The change is designed not to break any usage of the functions; C++17 guarantees that there's an implicit conversion from std::string to std::string_view. An exception are attempts to construct single-character strings from char.

• 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.
• 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).
• 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

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.11%. Comparing base (166a7d4) to head (ee9b0a7).

Files with missing lines	Patch %	Lines
src/util/string_hash.cpp	60.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9006      +/-   ##
===========================================
+ Coverage    80.55%   81.11%   +0.56%     
===========================================
  Files         1707     1707              
  Lines       189016   210291   +21275     
  Branches        73       73              
===========================================
+ Hits        152261   170585   +18324     
- Misses       36755    39706    +2951     

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