Change the `connect()` API to lift up the use of `connector()` to
`Endpoint::connect()`, allowing users to provide their own
implementations (for example, Unix-domain sockets).
Any type which impls `tower_make::MakeConnection` is
suitable.

To avoid breaking the default case of HTTP(S), introduce
`connect_with_connector()` and retain `connect()`, which creates the
default connector according to the activated feature gate and passes it
to `connect_with_connector()`.

Fixes: hyperium#136

…ated code

Instead of providing a connector in the call to `connect()`, make `Endpoint`
contain a connector, by default an `HTTPConnector`, which the caller can
swap.