Add new warning status and result state

[CharliePoole]

This issue is preliminary to all other work on the Warning-level Assertions Epic.

To do, while making sure all tests continue to pass:

• Create a warning TestStatus
• Create a warning ResultState
• Ensure that all code that switches on either TestStatus or ResultState takes the new values into account.
• Redefine the Ignored result state as a warning and change logic around ignored tests
• Create and Test Assert.Warn
• Create and Test Warn.If and Warn.Unless (same overloads as Assume.That)
• Ensure that warnings are all saved in the <assertions> element, with status Warning.

[CharliePoole]

Blocked while waiting for a decision on the issue of redefining ResultStatus.Ignore as a type of warning. The other issues under the Warning Assert epic all depend on this one.