Report file, line number along with failures

- Use callsite structs to pass references to the file and line
  number associated with an expectation.
- Have World keep track of the currently executing example.
- Have Example maintain a weak reference to its current test case.
  This allows Expectation to message that test case regarding failures.
  Note that this couples QuickExpectations to QuickCore. This is an
  architecture regression that should be addressed in the near future.

This commit is an improvement, but it still doesn't resolve issue #7.
Although failures are reported, causing the test suite to fail, Xcode
only displays a red line and failure message at the callsite for a
split-second, then the message disappears.