Turned vibe.web.rest into a package

There are various addition / enhancement to `vibe.web.rest` that can be done.
Some of them (handling of different origin of parameter, automatic generation of OPTIONS handler, custom serialization...) while still being discussed with @Dicebot , will require reasonable amount of code.
Thus, this commit split the existing `vibe.web.rest` into a package, including modules `client`, `server`, and `common`.
Besides the new `package.d`, and some import tweaking, no code has been changed.