goto-contracts: Separate front-end for contracts

[tautschnig]

This introduces a new binary to handle all steps related to contracts
processing, which previously was part of goto-instrument.

• 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.
• 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.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6526 (19a8a0e) into develop (bb59fa6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6526   +/-   ##
========================================
  Coverage    75.98%   75.98%           
========================================
  Files         1576     1578    +2     
  Lines       180850   180905   +55     
========================================
+ Hits        137411   137466   +55     
  Misses       43439    43439           

Impacted Files	Coverage Δ
src/goto-programs/remove_returns.cpp	98.15% <100.00%> (+0.02%)	⬆️
...smt2_incremental/construct_value_expr_from_smt.cpp	100.00% <100.00%> (ø)
...smt2_incremental/construct_value_expr_from_smt.cpp	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 198fd86...19a8a0e. Read the comment docs.

[kroening]

Does it make sense to include crangler, i.e., work at the source level, to deal with static?