Disabling all features means the only dependency is `futures`.
Relevant pieces of the API can then be enabled with the following features:
- `codec`
- `fs`
- `io`
- `reactor`
- `tcp`
- `timer`
- `udp`
- `uds`
This also introduces the beginnings of enabling only certain pieces of the `Runtime`. As a start, the entire default runtime API is enabled via the `rt-full` feature.
This patch keeps the primary net types in `tokio::net` and moves
secondary types to a protocol specific submodules.
Primary types are the ones that users are most likely to name (`TcpStream`,
`TcpListener`, `UdpSocket`, ...)
Secondary types are the operation futures.
Sean McArthur