chore: Entry points refactoring

[kulikthebird]

We should consider refactoring of the entry_points macro, since it wasn't aligned with the latest features and design changes implemented in Sylvia. Potentially there's a way to simplify the code for generating entry points.

Also currently Sylvia implicitely requires that #[entry_points] macro be used before the #[contract]. If there is a way to make the attributes order redundant it would be really beneficial for users, as it would prevent some problems with lack of generated entry points in contracts. For sylvia v1.1 and older there's no warning/error message if #[entry_points] is placed in "wrong" order which may result in a contract binary without exposed entry point methods.