Since the core Python `ChainMap.get()` implements with:

    self[key] if key in self else default

The double-chain-lookup is expensive.  This simple change solves that
for our ChainMap derived structure.  As such it improves matters
for #466 somewhat.

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

_yaml.py: Implement `get()` for `ChainMap`

See merge request !919