Fix a number of errors in loops

[Mingun]

This PR the result of revision of how generation of repeated attributes is implemented due to which a number of bugs was found and fixed:

• TypeValidator incorrectly define _ variable for all kinds of loops, but it is generated only for repeat-until loops
• fixes https://github.com/kaitai-io/kaitai_struct/issues/958, closes #245
• C#, Go, Java, JavaScript was calculate count of repetitions of repeat-expr loops on each iteration, whereas other languages performs such calculations only once. Now they are also calculate count only once
• Now Perl defines _index variable in repeat: eos loops
• Go and Rust was incorrectly used root IO object instead of current IO object of the attribute. The difference should be noticable when io: ... is used

This PR supersedes #234

Also, some refactoring was done: removed unused parameters and documented methods that generates loops.

Besides, _currentIteratorType was renamed to _lastParsedType (this is the variable that holds type of the _ variable in expression language) because it is used also in valid: expr key to refer to the element that just was parsed.

[transverberate]

Thank you for fixing this and closing the PR. I apologize for not following up on my PR last year. I'm happy we have this consistent behavior across languages.