Let's have panics with explanatory messages

[wkordalski]

Firstly, thank you for this crate!

Secondly, a cosmetic programmer-experience improvement:
The documentation clearly states that inserting an empty range into a RangeMap results in panic.
And it is checked with assert!().

It would be nice if the asserts get some messages, that explain that inserting an empty range is forbidden.
This would make debugging faster and understanding panics easier.