BaseFold: Add and reimplement some utility functions.

[yczhangsjtu]

Extracting small PRs from #294

There was an implementation of Iterator for iterating through a FieldType. Turns out that it can be replaced with existing tools in iter_tools. This proposes the new implementation.

Also added some other utility functions for type conversion between FieldType and Vec<E>, to avoid match field_type everywhere.

[matthiasgoergens]

Thanks for breaking down the big PR into more digestible chunks!

[matthiasgoergens]

Could you please add in the PR description why we are making this refactoring?

Just a quick note about what task becomes easier or simpler with the new organisation?

Thanks!