0.0.4

Bug fix:

• fix default slab size when sync feature is enabled
• fix panic in Deref/DerefMut for Slice extending into uninitialized part of the buffer
• fix stream_ext for latest nightly toolchain

Features:

• allow using shutdown to close connection write
• add async buf read trait and BufReader
• add File::read_exact_at and File::write_all_at
• use tracing instead of eprintln as debug output
• support UnixDatagram; allow split for unix stream
• change IoBuf and IoBufMut pointer semantics
• allow not Sized type for AsyncRentExts
• implement io copy util
• allow ReadHalf and WriteHalf reunite
• update iobuf and iovec trait

Thanks @18o @dragonly @pymongo @songzhi @ihciah and @dyxushuai for thier contributions!