Switch to fastlike.dev as the package name

Switch backends from RoundTrippers to Handlers

Originally, this project used http.Handler for the backend type, but I switched
to http.RoundTripper because I thought the general use-case would be sending
fastlike subrequests to another process.

While I still believe that to be generally true, having backends be Handlers is
simpler for embedders that want to respond in-process, and it's very easy to
create a reverse proxy to send requests upstream that still implements the
`Handler` interface.

Now with TLS support! (kind of)

With the addition of `SecureRequestOption` embedders can control whether a
downstream request should be considered "secure", and have the scheme set to
https and the `fastly-ssl` header set.

By default, a request is considered secure if it has a non-nil `TLS` property,
which is set by `net/http` when accepting a request over a TLS server and
before calling a handler.

An embedder may choose to also consider a request secure if it comes from an
upstream proxy that terminates TLS, in which case the secure option func may
return true if the request has an `x-forwarded-proto` header of `https`

Fix capitalization in import path