Performance tests are invoked via "sbt perf:test". Currently testing
performance of CEGIS on some random synthesis problems

the TerminationRegression test checks if counterexamples are found

counting from 31 down to 0 instead of from 0 up to 31

Functions that are defined as @extern can use non-purescala features
within their body. Leon will reason about them according to their
specification (the unknown body becomes a hole and then a choose).

Refactor all existing Modulo into Remainder, as it is
the correct term in Scala for the operation %.

BigInt now supports `mod` operator, which gets represented
as Modulo in Leon.

Limitations:
 - Test suites about Fairz3 need full refactoring to shutdown
 - No shutdown in termination checkers for now.