Move JSVal into the jsval module.
Use FooValue() functions.
Use the JSVal member functions to replace JSVAL_IS_* and JSVAL_TO_*.
Update rust-mozjs to include fixes for GC crashes.
auto merge of #1868 : Ms2ger/servo/JSVal, r=jdm

Requires https://github.com/mozilla-servo/rust-mozjs/pull/54.