Improvements and bug fixes for sizeof calculator

[Mingun]

This PR improves situation with _sizeof / sizeof<>:

• switch-on types with the same size in all arms now correctly have the constant size
• if keys now correctly influence to class / attribute sizeof (probably this will fix https://github.com/kaitai-io/kaitai_struct/issues/927)
• as a side effect, improvements in the constant evaluator of boolean values
• add tests for boolean constant evaluator
• add tests for sizeof calculator
• forbid explicit negative repetition number in the repeat-expr key
• forbid explicit repeat-until: <falsy value> because that will lead to an infinity cycle

This PR is based on the #225 because the last commit (f6238c0d943325c3a5e81c9523ba1a214318981a) from that PR is needed for correct switch-on size calculations

Also this PR fixes the expr_sizeof_value_dynamic3.ksy test -- corresponding PR to the tests repository: https://github.com/kaitai-io/kaitai_struct_tests/pull/95. Also, this test was mentioned in the https://github.com/kaitai-io/kaitai_struct/issues/721

[Mingun]

@generalmimon , I see that you've started to merge some PRs and close issues. Could you look at my PRs? This one is a good candidate to start. List of all my PRs: https://github.com/pulls?page=1&q=is%3Aopen+is%3Apr+author%3AMingun+archived%3Afalse+org%3Akaitai-io

[Mingun]

@GreyCat, @generalmimon, I see that you have some activity in the project recently. Could you find a time to review my PRs, for example, this?