Add configurable event storage

[CatThingy]

Objective

• Fix Allow opt-in configuration of events to use SmallVec #2071
• Adoption of Configurable Events<T> storage #2073

Solution

• Add an associated type on Event for the storage type to use

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Changelog

• Added the ability to let events be stored in a SmallVec

[james7132]

Thinking a bit harder on this, I want us to have quantifiable metrics via #7731 for this. The extra branch on the configuration (if we can't eliminate it) is a constant cost regardless of whether we use smallvec or not. If the memory saving benefits are more than worth the CPU cost, I'd say we should still merge.