Implement AsRef and Into conversions between tide Requests/Responses and their inner http_types equivalents

[jbr]

this is the tide mirror issue of http-rs/surf#167
this allows for an escape hatch to drop down to apis that might not need to be exposed at the tide level. this would also allow for better interop between surf and tide, such as providing concise support for proxy operations (receive a surf::Response, unwrap it into an http_types::Response, and re-wrap that as a tide::Response)

• tide::Request -> http_types::Request
  • Into
  • AsRef
• tide::Response -> http_types::Response
  • Into
  • AsRef
• http_types::Response -> tide::Response (Into)
• http_types::Request -> tide::Request (Into)