Clarify cast rules (tracking issue for RFC 1052)

[nikomatsakis]

Much of the implementation work for rust-lang/rfcs#1052 has been done, but there remain some corner cases to fix:

• Casts between raw pointers to different traits are accepted but should not be (e.g. http://is.gd/1jCOSz)
• Double-check other cases, ensure adequate tests exist

[nikomatsakis]

triage: P-high -- I'm giving this high priority since accepting casts that we ought not to is a backwards compatibility risk, though it is mitigated by the fact that afaik the illegal casts yield an ICE (but I haven't done exhaustive testing)

[arielb1]

The newly-illegal casts shouldn't be ICE-s (I mean, on nightly). Casts on stable are just randomly ICE-ey.