Remove unused import of std::intrinsics::uninit.
Remove the preSuccess and postSuccess arguments to getJSToNativeConversionTemplate.

They were used for unions until 06f9afdad52cba426aedd45d0e08a7b2bdecc8f4.
Remove the codeToSetNull argument to wrapObjectTemplate.
Use CGIfElseWrapper in a few more places.
Remove the declareResult argument to CGCallGenerator's constructor.
Make the result and result_fallible variables immutable.
Stop returning None from getRetvalDeclarationForType and update documentation.
auto merge of #1949 : Ms2ger/servo/cleanup-codegen, r=jdm