Also use `auto` and structured binding for iterators.

This is based on running `clang-tidy` with the `modernize-loop-convert`
and `modernize-use-auto` checks. Note though that I did not (yet) apply
any `auto` suggestions that were not `for` loops.

This uses clang-tidy's `modernize-use-nullptr` check.

With `emplace_back`, we don't need to create an object ourselves, to be
copied into the container, plus it returns a reference to the new object
for us to use.

This used clang-tidy's `modernize-use-emplace` check, plus some
followups to use the return values.

This is applying clang-tidy's `modernize-use-equals-default` and
`modernize-use-equals-delete` checks.

This applies clang-tidy's `modernize-type-traits` check.