Reorder implementations in conversions.rs.
Extend and update primitive conversions.
Use JSValConvertible::to_jsval from codegen.
Implement support for returning nullable primitive types.
Add tests for optional primitive arguments.
Rewrite the codegen for nullable primitives to use JSValConvertible.
auto merge of #1814 : Ms2ger/servo/more-primitive-setters, r=jdm