This splits up `tests/ui/checked_unwrap.rs` into:
* `tests/ui/checked_unwrap/complex.rs`
* `tests/ui/checked_unwrap/simple.rs`
Based on the naming of the methods in the tests.
cc #2038
This checks for things like
if x.is_some() {
x.unwrap()
}
which should be written using `if let` or `match` instead.
In the process I moved some logic to determine which variables are
mutated in an expression to utils/usage.rs.
