Add multi_index::require_find

[arhag]

Since multi_index::get can easily be misused as documented in issue #4226, contract developers should have an alternative that is safer.

That alternative already exists. They can use multi_index::find to get a const_iterator and then pass that const_iterator into modify or erase.

But a common pattern is to check that the iterator actually points to a real object (meaning the table row was found) and throw an assertion error otherwise. In fact, get makes this very convenient by taking an optional string argument with the assertion message to throw if the table row cannot be found.

multi_index::require_find would behave similarly to get in the sense that it asserts that the requested table row was actually found and uses the optionally provided message string as the assertion failure message. However, it would return the safe const_iterator just like find does.