Fetch improvement project

Background information: Servo currently supports two ways of obtaining HTTP data from the network. The first is the original implementation that was based loosely on Firefox's network stack; the second is new and implemented based on the Fetch specification. We are in the process of transitioning from the old to the new, and the goal of this project is to support that effort by implementing some missing pieces in our Fetch implementation.

Goals:

• implement missing HSTS step in the main fetch algorithm
  • this should be able to reuse the existing HSTS implementation used by http_loader.rs (old stack)
• implement SRI support (more info), integrated with the main fetch algorithm
  • this is brand new and needs to be integrated with main_fetch in methods.rs
• implement missing handle fetch step for HTTP fetch algorithm
  • this should use a similar design to the existing implementation in http_loader.rs (old stack), integrate with http_fetch in methods.rs
• if time permits, convert some more callers from old network stack to new Fetch implementation
  • eg. htmlmediaelement.rs, htmllinkelement.rs