Limit spans in bytes!() error messages to the argument in question

This constrains the span to the appropriate argument, so you know which
one caused the problem. Instead of

  foo.rs:2:4: 2:21 error: Too large integer literal in bytes!
  foo.rs:2    bytes!(1, 256, 2)
              ^~~~~~~~~~~~~~~~~

it will say

  foo.rs:2:14 2:17 error: Too large integer literal in bytes!
  foo.rs:2    bytes!(1, 256, 2)
                        ^~~
std/rt: in-progress file io work

std: remove unneeded field from RequestData struct

std: rt::uv::file - map us_fs_stat & start refactoring calls into FsRequest

std: stubbing out stat calls from the top-down into uvio

std: us_fs_* operations are now by-val self methods on FsRequest

std: post-rebase cleanup

std: add uv_fs_mkdir|rmdir + tests & minor test cleanup in rt::uv::file

WORKING: fleshing out FileStat and FileInfo + tests

std: reverting test files..

refactoring back and cleanup...
merge cleanup
std: adding file::{stat,mkdir,rmdir}, FileInfo and FileReader/FileWriter

add ignores for win32 tests on previous file io stuff...
std: win32 os::env() str parsing to str::raw::from_c_multistring + test
std: clean up Dir/FileInfo inheritence and flesh out Dir Info
std: more work on from_c_multistring.. let it take an optional len param
std: expose more stat info
std: bind uv_fs_readdir(), flesh out DirectoryInfo and docs/cleanup
std: unignore some file io tests that work on windows, now
std: correctly pass STDOUT in to naive_print test fn
std: generlize & move io::file::suppressed_stat to io::ignore_io_error
std: remove impl'd/commented-out fstat signatures
std: FsRequest.req_boilerplate() be &mut self
std: lots of docs for std::rt::io::file

i hope they don't bitrot
std: minor cleanup in some io_error descs in io::file
std: docstring fixes in io::file
std: merge conflict cleanup from std::str
std: remove RtioStream
std: fix win32 build error in os::env()