Add core::cmp::equiv to prelude

[Aatch]

As mentioned in https://mail.mozilla.org/pipermail/rust-dev/2013-April/003867.html, equiv should probably be imported by default.

Related: #6145

Nominating for maturity 1.

[graydon]

For future reference: to nominate for a milestone, mention the milestone you think it belongs on and tag the bug with I-nominated. We go through the nominations once a week and either accept or decline them.

I've adjusted this accordingly.

[graydon]

accepted for feature complete

[Aatch]

fixed by #6216