More testing, new failing test for JsonWithDeepLinks

Better performance test harness for `IMediaTypeHandlers`

Fix mediaTypeHandler tests to use `.Write<IEnumerable<X>>()` where we would expect `IOutput<IEnumerable<X>>`

`LinkConverter` now doesn't barf when passed an `IEnumerable<>` interface directly.

JsonWithDeepLinks, links in the entire hierarchy still do not work.

ExplicitTestConverters will write full graph now, on par with other converters