Use Collection.assertEqual instead of 'assertEqual' in a few more places to get better error messages for failing tests.
Small improvements to the .ToString() implementations for IntSet/IntMap/LongSet/LongMap which avoid some unnecessary allocations.
Fixed the 'checkThrowsExn' function used by the tests imported/adapted from FSharp.Core; now, we don't accidentally capture the exception raised by Assert.Fail(), which is called when the function-under-test *does not* throw an exception as expected. This means we get the correct error message when the test fails, instead of a message about the exception being the wrong type of exception.