element.py: Alter Element.__calculate_cache_key() to pre-santize inputs

In order to reduce the effort we spend in `_yaml.node_sanitize()` take
advantage of how `OrderedDict` works and pre-sanitize the majority of
the cache-key once.  This approximately halves the amount of effort we
spend in `_yaml.node_sanitize()` in pre-scheduler build scenarios.

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