…tself

Also, don’t check `DefaultIndices` — it’s fine to
assume it implements Collection as long as the base collection is
correct.

…ex(_:offsetBy:limitedBy:)

…ild in a non-unique node

Oops, this did not account for reversed item order, so it was spawning the new child with the wrong item.

This will simplify merge operations as we no longer have to spend effort on preventing repeated rehashings of items within collision nodes.

Also, review and simplify node allocation and lookup code.

…ubset/isDisjoint

The new executable target `memory-benchmark` collects some memory usage statistics and outputs average memory efficiency for a range of sizes in CSV format.

Bump required toolchain version for benchmarks to Swift 5.5.

…StringConvertible, CustomReflectable conformances

… (untested)

This adds an experimental _FastMembershipCheckable protocol to speed
up these relationship predicates when possible — having separate
overloads for each known collection with a fast containment check
is not going to scale for long.

This commit also introduces a set of isEqual(to:) methods that
generalize ==.

Plus add tests covering these as well and mapValues.

…ring (untested)

…ns (untested)

…istentSet and a value transform