convenience methods in __init__.py should modify FullLoader instead of Loader

[dthkao]

For example add_constructor() in 5.1 is a no-op for legacy code since it adds the constructor to Loader but FullLoader is the one used in load(input).

https://github.com/yaml/pyyaml/blob/master/lib3/yaml/__init__.py#L329-L344

[perlpunk]

Thanks, we're discussing this. It will always break some code, but since the default for load is FullLoader that seems to be the best.

[perlpunk]

I created #274

[perlpunk]

This was fixed in 5.2