Add ok_expect lint

[BrunoAmbricca]

What it does
Checks for usage of ok().expect(..).

Why is this bad?
Because you usually call expect() on the Result directly to get a better error message.

Known problems
The error type needs to implement Debug

Example
x.ok().expect("why did I do this again?");

Use instead:
x.expect("why did I do this again?");