Implement task-safe platform-indepent timezones

[klutzy]

Similar to #10463, extern::time::at() (which internally uses localtime()) currently depends on "TZ" environment variable, which causes setenv race.
Also its interpretation is also platform-dependent: linux libc understands TZ=America/Los_Angeles but Windows does not. (#10307)

We need:

• Timezone type whose constructor may understand common time zone strings.
• fn localtime(&self, utc_time) -> Tm method for Timezone. (This seems quite hard because it seems that there is no corresponding libc function)
• For environment timezone, we may provide unsafe fn get_env_tz() -> Timezone.

I think this overlaps datetime design a bit, but anyway current implementation is "unsafe".

[huonw]

cc #2153, #2637.

[frewsxcv]

The time module has moved to a separate repo https://github.com/rust-lang/time Should this Issue be moved there or is this an internal rustc component?

[klutzy]

Yes, it's moved to time crate. (time::at() now)

[frewsxcv]

Can this be closed then?

[Gankra]

At best, this would be an RFC issue anyway.