[ENH] Move performance metrics to benchmarking

[TonyBagnall]

I dont feel strongly about this, its ok for them to remain where they are if anyone does, but on balance I favour keeping the root as slim as possible and encapsulating where we can, performance_metrics seems better in benchmarking to me.

update: pycharm refactoring is so bad!

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[TonyBagnall]

refactoring too tedious