Updates to 1.66 and added assertion messages.

Assertion messages need to be improved such that the user is made aware which is expected and which is actual. This is particularly important for beginner level kata.

I've just bumped the version

"Language Version is invalid for rust"

• Updated to Rust 1.50
• Updated fn signature for rand Rng::gen_range()
• Added random tests with a minimum string length of 1

There's no need to declare result if you are going to return it right away; also, there should be at least one sample test case like in the other language katas.