libtime::Tm::to_utc() is slow even if already utc

[seanmonstar]

Tm.to_utc() calls out to the runtime fn rust_timegm twice, once to convert to a Timespec, and a second time to convert the timespec to a new Tm with offset of 0. If a Tm is already UTC, this should skip both those calls.

[reem]

rust_timegm is also very slow, perhaps curiously so. It shows up as almost 60% of the running time of hyper for a hello world server.

[gsingh93]

This should probably be closed or moved to https://github.com/rust-lang/time

[steveklabnik]

Done! time-rs/time#66