These lemmas were inconsistently marked as `@[simp]`, but they seem
generally useful, so this uniformly marks this lemmas as `@[simp]` for
all map variants.
This PR adds extensional hash maps and hash sets under the names
`Std.ExtDHashMap`, `Std.ExtHashMap` and `Std.ExtHashSet`. Extensional
hash maps work like regular hash maps, except that they have
extensionality lemmas which make them easier to use in proofs. This
however makes it also impossible to regularly iterate over its entries.
