Support arbitrary stdout/stderr/logger handles

[alexcrichton]

This will allow capturing of common things like logging messages, stdout prints
(using stdio println), and failure messages (printed to stderr). Any new prints
added to libstd should be funneled through these task handles to allow capture
as well.

Additionally, this commit redirects logging back through a Logger trait so the
log level can be usefully consumed by an arbitrary logger.

This commit also introduces methods to set the task-local stdout handles:

• std::io::stdio::set_stdout
• std::io::stdio::set_stderr
• std::io::logging::set_logger

These methods all return the previous logger just in case it needs to be used
for inspection.

I plan on using this infrastructure for extra::test soon, but we don't quite
have the primitives that I'd like to use for it, so it doesn't migrate
extra::test at this time.

Closes #6369

[alexcrichton]

Updated with comments (other than adding warn! statements)