RFC: Coercible and HasPrefix for Zero Cost Coercions

[rust-highfive]

This is largely based on (with lots of copied text) @glaebhoerl's proposal in rust-lang/rust#9912, but with a few changes:

• I formalized the idea of "contexts" by using GHC's roles.
• I used this role system to include user-defined pointers and data structures.
• I dropped contravariance for simplicity. This may have been a bad idea, but it's unclear how useful contravariance is.

Tracking issue for postponed PR #91

[glaebhoerl]

cc #223

[glaebhoerl]

cc #1387