Add `snapshot` to `MapRef`

[iRevive]

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Motivation

MapRef can be considered as a replacement for Ref[F, Map[K, V]]. However, currently, there is no way to retrieve all available values.

Why not add snapshot method directly to the MapRef trait?

Two reasons:

• We cannot implement snapshot for the def fromSeqRefs[F[_]: Functor, K, V] (traverse requires Applicative)
• For the majority of the implementations, the result type would be Map[K, Option[V]], which is suboptimal