libterm: Use Windows Console API to color texts.

[ghost]

This patch adds the coloring function on Windows console to libterm.
Fixes #13400, and makes working on MSYS slightly more pleasant.

Quick Notes

• Extract trait TerminalOps<T> from Terminal<T>.
• Add struct WindowsConsole<T> along with Terminal<T>, both implements TerminalOps<T>.
• term::Terminal::new is now term::new_terminal, returns ~TerminalOps<T>.
• Windows-specific code, including WindowsConsole<T>, is in the module windows.
• It will assume it's running on terminal that understands ASCII if $TERM is "xterm". Attempts to not break rxvt.exe.
• It spams :Send to deal with libsyntax, which packs Terminal into a RefCell. I couldn't come up with any smarter fix.

I had to remove Terminal::unwrap() to implement Drop on WindowsConsole<T>, thus it lose the way to manually destroy itself. I suppose it's okay, since I couldn't find any use of it in the tree.
Also, it will not be possible to access the members of Terminal<T>, such as TermInfo. It's still possible to see terminfo by calling term::terminfo::parser::compiled::parse, though.

As this is Windows-only improvement, I'm not sure about breaking change is acceptable. I can consider more humble way to do this, if it doesn't pay.

[klutzy]

(There is similar patch on #13401 by @cmr.)

I think #13400 is about providing wrapper for low-level apis like SetConsoleScreenBufferSize. But I don't object closing it since I don't think we need them in near future.

[ghost]

Oh, okay. Should I close this?

[emberian]

Good to know that our implementations converged (more or less). Thanks for working on this, sorry I've been slacking with that other PR :(

[adrientetar]

~ was removed from the language.

I guess the question is which one of these patches works best.

[ghost]

I'd go for @cmr's one, which is more organized and has better documentation.
It's amusing to see we both took essentially same approach. Nice work! :)