This change starts plumbing ASP.NET Core HttpContext through the HttpContext
and sets up the caching mechanism used to ensure only a single instance
of the HttpContext is created per context.

Conversion between the adapters and core types are provided via both
implicit operators as well as extension methods that get or unwrap the
adapters. The HttpContext adapter is stored in the core
HttpContext.Features so that only a single instance is used for each
instance of the core context. The additional members (such as .Request
or .Response) are cached within the adapters and only created on demand.

This change also includes the HttpContextWrapper, HttpRequestWrapper,
and HttpResponseWrapper which are converted and stored in a similar way
as to the HttpContext adapter.