Simplify program entrypoint (anza-xyz#166)

* Fix review comments

* Revert offset increment change

* Add invoke instruction helper

* Typos

* Remove new helpers

* Remove unused

* Address review comments

* Tweak inline attributes

* Use invoke signed unchecked

* Refactor inline

* Renamed to with_bounds

* Update docs

* Revert change

* Add constant length check

* Simplify accounts deserialization

* Invert borrow state logic

* Use expr instead

* Add missing import

* Address review comments

* Revert unnecessary repr

* Fix rebase

* Tweak docs

* Fix doc reference

* Fix miri errors

* More review comments

Author: febo