2.4.0 - various fixes for test ids and lazy values

• is_lazy is now part of public API, and _LazyValue now has a cache mechanism like _LazyTuple. Fixes #143

• @parametrize: custom ids are now correctly taken into account when a single lazy_valueis used for a tuple of parameters. This issue could be seen also with @parametrize_with_cases: idgen does not seem to be taken into account when cases are unpacked into a tuple. Fixes #144.

• Empty case ids are now replaced with '<empty_case_id>' to avoid ambiguous interpretation of test ids. Fixes #142.

See documentation page for details.