Redesign the I/O library and traits

[brson]

It is desperately needed. Some requirements:

• Default implementations must not ever block the Rust scheduler
• Should support posix streams, files, sockets, etc.
• Should support streams built around uv (non blocking)

Work items:

• Design and implement I/O error handling strategy #6163 Error handling
• String encoding/decoding and I/O #6164 String encoding/decoding
• Implement non-blocking TCP #6165 Non-blocking TCP
• Benchmark single-threaded, non-blocking TCP performance #6166 Benchmark single-threaded TCP performance
• Non-blocking File
• Non-blocking UDP
• Non-blocking Unix
• Non-blocking stdio
• Implement blocking file I/O using new I/O subsystem #6168 Blocking File
• Platform-backed, non-scheduler/runtime entangled IO #9488 non-scheduler/runtime-coupled IO
  • Blocking TCP
  • Blocking UDP
  • Blocking Unix
  • Blocking stdio
• Replace core::io with blocking implementation of core::rt::io #6169 Remove old core::io
• DeflateStream
• Memory streams
• Implement HTTP client and server library #6167 HTTP
• Pipe adapters
• Base64 adapters
• Add timers to core #6435 Timers
• Add a uv backend to core::run #6436 Reimplement core::run
• Iterators and I/O
• Need a solution for select / async events #6842 async i/o
• Add buffered I/O #8953 buffered I/O
• DNS name resolution more than getaddrinfo() #8925 DNS

Completeness

• io::file needs to get expanded to be "fully functional" #10131 finish io::file
• io::FileStat needs to be less platform-dependent #10130 FileStat needs polishing.

[brson]

Related to #4419

[brson]

Some useful comments in #2004