SYNTHESIZER: Migrate loop-contracts synthesizer from goto-instrument into goto-synthesizer

[qinheping]

This PR copies the synthesizer interface and expression enumerator from goto-instrument into goto-synthesizer, and removes them from `goto-instrument.

[codecov]

Codecov Report

Base: 78.39% // Head: 78.38% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (2a745f8) compared to base (b03d870).
Patch coverage: 85.71% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7429      +/-   ##
===========================================
- Coverage    78.39%   78.38%   -0.02%     
===========================================
  Files         1655     1659       +4     
  Lines       190281   190372      +91     
===========================================
+ Hits        149172   149217      +45     
- Misses       41109    41155      +46     

Impacted Files	Coverage Δ
.../goto-instrument/goto_instrument_parse_options.cpp	71.31% <ø> (-0.17%)	⬇️
...rc/goto-instrument/goto_instrument_parse_options.h	100.00% <ø> (ø)
src/goto-synthesizer/expr_enumerator.cpp	75.52% <ø> (ø)
src/goto-synthesizer/expr_enumerator.h	94.59% <ø> (ø)
src/goto-synthesizer/synthesizer_utils.cpp	92.85% <ø> (ø)
...to-synthesizer/expr_enumerator/expr_enumerator.cpp	100.00% <ø> (ø)
...hesizer/enumerative_loop_contracts_synthesizer.cpp	87.50% <66.66%> (ø)
...oto-synthesizer/goto_synthesizer_parse_options.cpp	67.34% <85.00%> (ø)
...nthesizer/enumerative_loop_contracts_synthesizer.h	100.00% <100.00%> (ø)
...goto-synthesizer/loop_contracts_synthesizer_base.h	100.00% <100.00%> (ø)
... and 21 more

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[tautschnig]

I believe the man pages are also to be updated, and the same is true of the CODEOWNERS file.