- clean up trace macro some

Apparently gmtime was already implemented when I made localtime, so we had two different things written from scratch. We decided in the relibc channel of the Redox OS Mattermost chat to use my code, as it is more extensively tested and perhaps is clearer in how it works.

Add contribution guidelines/tutorial

See merge request redox-os/relibc!162

Updated url, implemented missing functions in header/arpa_inet/mod.rs

See merge request redox-os/relibc!161