auto merge of #1861 : Ms2ger/servo/split-JSValConvertible, r=jdm

Later, we'll want to implement variations of the conversion *from* JS, but
not the conversion *to* JS.
Completely rewrite the code to convert JSVals to unions.

This code works correctly for the two unions currently in use, and raises
exceptions in codegen for most other cases. I will implement those cases when
the need arises.