Remove cross-borrowing entirely

[pcwalton]

Tracking bug for RFC PR rust-lang/rfcs#139.

Nominating for 1.0, P-backcompat-lang.

[lilyball]

This needs to be blocked on DST. @wycats pointed out in IRC that this breaks the ability to coerce Box<Trait> to &Trait. The expected solution is to use &*x, but that's not legal until DST lands.

[lilyball]

Summary of IRC discussion:

@pcwalton feels that transmute() is an acceptable solution. Others disagree. @nick29581 says that the part of DST that allows this is estimated to land in ~2 weeks. @pcwalton has agreed that ~2 weeks is an acceptable amount of time to delay this.

[pcwalton]

Note that, per later discussion, this doesn't have to block cross-borrowing entirely, just cross-borrowing for traits.

[pnkfelix]

Assigning P-backcompat-lang, 1.0 milestone.