Implement `CoerceUnsized` for `Unique`
Implement `CoerceUnsized` for `Unique`

Closes rust-lang/rfcs#1343.

GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust