New combinator: `multi::reduce`

[bertptrs]

This PR adds a new combinator: multi::reduce. It is essentially a special case of multi::fold_many1 where the accumulator type is the same as the parsed type.

Analogous to Iterator::reduce, this new combinator takes the first parsed element as the initial value rather than use a separate initializer function. This is useful in many cases, but most commonly for when you do not have an easy base element for your reduction, for example when computing a minimum over a list of elements without a well-defined absolute maximum.

Aside from this new combinator, the PR also includes the following changes (in separate commits for easy review):

• Fix a test for a specific struct size. Apparently rustc got smarter?
• Refactor shared code between fold_many0 and fold_many1
• Breaking: change the infinite loop error for fold_many1 from Error to Failure. I believe this was always the correct version since it indicates a programming error and not a data issue.

Tests for new code included.

[bertptrs]

This PR is now almost a year old and has various conflicts with the main branch so it cannot be merged but I would still like the combinator added here.

Is this something that could be added? If so, I'll clean up the PR so that it works again.