Implement blocking file I/O using new I/O subsystem

[brson]

The new scheduler won't be ready for a long time, but we can replace core::io with a blocking implementation of the new I/O API. Need to make the implementation first though.

[emberian]

Newsched is in a really good place and std::rt::io::file has some stubs, marking as E-easy.

[olsonjeffery]

Basic file io actions are implemented in std::rt::io::file and the groundwork is laid for future expansion. What is the "definition of done" for this ticket? (fwiw, I'm moving-on to immediately begin impl'ing remaining functionality).

[alexcrichton]

Basic functionality is now provided in std::rt::io::native::file. Currently there is no support for doing anything other than reading/writing existing handles. Items left to-do:

• Creating handles (FILE* and fd_t)
• Removing files
• Creating directories
• Removing a directory
• Basic stat information: get a stat, test what a path is, test if a path exists, etc.

These may just be nice wrappers around libc functionality, but they should be officially re-exported under std::rt::io::native::file as necessary.

[doy]

This appears to have been fixed in 68d5510.

[alexcrichton]

Good point, thanks!