Add *throw-mismatches* as a sort of hacky debug mode
Improve, test, and document the *throw-mismatches* feature.

It's now documented what *throw-mismatches* does, and the documentation
includes dire warnings about misusing it. Additionally, there is a new
function, mismatches, which searches a schema for all mismatches, rather
than relying on the programmer to scan through a schema by eye for
problems.