Implement fold and reduce methods for tables #118
Description
Right now it's impossible to iterate over the records and collect some data out of them as iter_with takes Fn as a function, so no mutations. Replacing Fn with FnMut might not be the best solution as in some cases the function shall not mutate anything. That's why we should consider providing fold and reduce methods.
As for now it's possible to workaround the limitation via Cell.