Defer error message evaluation for swift matchers

[firesteps]

• Defer error message evaluation until expectation fails.
  • Matchers and Sync/AsyncExpectation evaluate error messages eagerly which leads to String(describing:) calls even when expectation passes. In modules with a large number of types this call is expensive. In some of our modules the test run profiling shows 30-90% of CPU time is spent in String(describing:) for the work that's discarded when expectation passes. The largest improvement I've measured is a test suite run dropping from 60s to 5s.
• Error messages are not changed.
• Add tests to check if String(describing:) was called by introducing type with a CustomStringConvertible implementation.
• PR doesn't affect public API and doesn't change Objective-C matchers

Important

Linux support is not tested due to #981