Enable Specifying Dependencies of `serve`

Currently, several bounds of the serve function rely on private traits and types which aren't exposed from hyper. This makes it impossible to create a function in another library which accepts a compatible Executor type and passes it onto the serve function. Would it be possible to either expose these traits, or expose the Exec enum which implements them, so that libraries which wrap hyper can pass on this dependency to their users?

Start with the serve bounds in src/server/mod.rs around the linked location, then inspect the Exec enum in src/common/exec.rs and its implemented traits. Determine which public API would let wrapper libraries pass a compatible executor to serve; done means an external library can express and forward that dependency without relying on private hyper items.