Rewrite the interface unwrapping to return an expression.
Rewrite the union unwrapping to return an expression.
Rewrite the DOMString unwrapping to return an expression.
Rewrite the any unwrapping to return an expression.
Rewrite the primitive unwrapping to return an expression.
Rewrite the enumeration unwrapping to return an expression.
Move responsibility for assigning to ${declName} out of getJSToNativeConversionTemplate.

This commit does not make any change to the generated code.
Remove the 'retval' local variable from union TryConvertTo methods.
Rewrite dictionary codegen to initialize members directly into the struct.
auto merge of #2228 : Ms2ger/servo/object-unwrapping-expressionify, r=jdm