- `Grpc_protobuf` is no longer eio specific

- Make use of ocaml-protoc's server implementation bundle as intended;

- Adapt routeguide example to ocaml-protoc to show the differences.

This results in a more complicated [Grpc.Rpc] interface that tries to
better capture the common parts between ocaml-protoc and
ocaml-protoc-plugin that can be used by ocaml-grpc.

This reverts commit 042efeb.

- This adds type safety to ensure the RPCs are called with the
  expected protocol (e.g. you cannot call a unary rpc with a
  server_streaming entry point, etc.).

On the ocaml-protoc-plugin side, currently there are no markers for
the rpc modes - this interface will permit adding them in the future
without user facing changes.

On the ocaml-protoc plugin, the value mode flows from the proto file
definition and is checked in the user code as expected.

Implementation note: There's perhaps a way to shorten the mapping of
value-modes but I couldn't find one given that `Grpc` cannot depend on
`Ocaml_protoc`, and thus the `Value_mode` types are not equal.

In the existing examples, the service name is separated from the
package name by a dot, which I inadvertently omitted in the previous
implementation.

Note that as long as the service name used by a client and a server is
the same, the right handler is executed, so there's some leeway in the
actual choice of the convention to use. The hope is that the dot
separated one is standard.