Add diagnostic analyzers for Cesil conventions

[kevin-montrose]

Addresses #17

• [x] AsyncTestHelper.IsCompletedSuccessfully() is used instead of (Value)Task().IsCompletedSuccessfully
• [x] AwaitHelper.ConfigureCancellableAwait() is called for all awaitables prior to them being await'ed
• [x] BindingFlagsConstants is used instead of BindingFlags
• [x] Every use of the null forgiving operators (ie. suffix-!) is annotated with an explanation
• [x] Members on Throw are called instead of a throw statement or throw expression
• [x] Members on Types are used instead of a typeof() expression
• [x] Non-public types don't have public members
• [x] Analyzer performance needs some work
• [x] Tests for the analyzers