Multierror: join errors at the end (#132)

This fixes `MapErr` so that it does not create recursive `joinError`s.
By repeatedly calling `errors.Join(`, we create a nested set of
`joinError`s rather than a single, flat `joinError`. Then, when
`Error()` is called, it allocates a new string for each nested error
because `joinError` calls `Error()` recursively on each of its children.

Instead, this PR updates `MapErr` to just collect a slice of errors and
return the flat joined error.