We swap the arguments for `Membership.mem` so that when proceeded by a
`SetLike` coercion, as is often the case in Mathlib, the resulting
expression is recognized as eta expanded and reduce for many
computations. The most beneficial outcome is that the discrimination
tree keys for instances and simp lemmas concerning subsets become more
robust resulting in more efficient searches.
Closes `RFC` #4932
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Co-authored-by: Kim Morrison <kim@tqft.net>
Co-authored-by: Henrik Böving <hargonix@gmail.com>
