Use core not std

[Ericson2314]

I believe everything in the logging interface just requires core, not std. (The Box<Log> could become a Unique<Log> as it is never deallocated.) This is important for allowing the interface to reach its full potential, e.g. serial port debugging before allocation is set up. https://github.com/rust-lang/bitflags was similarly changed to use core for the same reason.

If this sounds good, I would be happy to do the implementation work.

[Ericson2314]

Ah, missed the rt::at_exit call. Perhaps the use of std, rt, and Box could be controlled by a cargo feature?

Edit: there is also eq_ignore_ascii_case but that doesn't need allocation or anything, so it could be moved defined elsewhere than std?

[Amanieu]

I submitted a PR for this: #68