Factor out charAt() and startsWith() simplification into separate functions

[danpoe]

The diff for this PR is a bit unhelpful. The PR simply moves the code that implements simplification of charAt() and startsWith() from simplify_function_application() to simplify_string_char_at() and simplify_string_startswith(), respectively.

• 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).
• 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.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 9503c49).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/118970054

[codecov-io]

Codecov Report

Merging #4898 into develop will increase coverage by <.01%.
The diff coverage is 91.11%.

@@             Coverage Diff             @@
##           develop    #4898      +/-   ##
===========================================
+ Coverage    69.19%   69.19%   +<.01%     
===========================================
  Files         1307     1307              
  Lines       107951   107955       +4     
===========================================
+ Hits         74696    74700       +4     
  Misses       33255    33255

Impacted Files	Coverage Δ
src/util/simplify_expr.cpp	87.28% <91.11%> (+0.05%)	⬆️

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[tautschnig]

@danpoe Could you please rebase this one so that the spurious CI failure (failure to parse an APT key, should be transient) is resolved?

[allredj]

Not particularly easy to review, because the diff is so weird, but it looks basically alright.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: ca203b9).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/119392278