_yaml.py: Remove use of isinstance() in `{node,list}_{chain_,}copy`

A non-trivial proportion of time pre-scheduler in `bst build` is spent
copying (or chain-copying) nodes.  Approximately a quarter of the time
spent in that effort is in `isinstance()`.  This removes that CPU load.

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