[stdlib] Transition `InlinedFixedVecor.type` to `CollectionElement`

[rd4com]

This commit is a possible transition for InlinedFixedVector,

changing it's type parameter from AnyTrivialRegType to CollectionElement.

It is for a feature request by @joeloser:

https://github.com/modularml/mojo/issues/3478

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Not sure if it is exactly what you had in mind @joeloser,

but here is a possible implementation :+1: