builtinMapFn makes unchecked call to toFunctionUnsafe

[nairb774]

One of the test cases passes None to the map builtin which is run through toFunctionUnsafe without first checking that such a conversion is safe. I discovered the unsafe cast when rebasing #58 to head.

[trotterdylan]

Good find. Not sure how I overlooked that during review. Fixed in c8d72d9.

[S-YOU]

Thanks for the report and fixes, I actually didn't know *Object can call it directly.