Crude implementation of Unicode console support for Windows.

This implementation does have the minor issue of not handling things correctly when a codepoint is split across multiple writes or reads, but its better than not having unicode support at all.

Adds a Windows specific struct `WindowsTTY` in `libnative` and make `tty_open` create that struct on Windows.. Also adds `ReadConsoleW` and `WriteConsoleW` to `libc`.

This will fix #15028 for libnative.

![](http://puu.sh/9BO4c.png)
`GetConsoleMode` merely tells us the set of flags that we've set for the console, not whether it is or isn't a TTY. `isatty` does the job fine at telling whether we're attached to a TTY or just redirected to a file.