_yaml.py: Reduce use of `isinstance()` in `node_sanitize()`

We know that nodes are typically one of:
str, list, dict, bool, tuple, NoneType or our ChainMap
Of these, dict and ChainMap are Mapping, only list is list
and the rest are returned unchanged.  We can reduce/defer our use
of isinstance here, dramatically, improving performance.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>