[Lint] Reject duplicate cases in switch-statement

[SupunS]

Issue To Be Solved

Currently, duplicate cases are allowed in switch-statement. That can leave unreachable codes without the user being aware.

e.g.:

pub fun test(s: String) {
    switch s {
        case "foo":
            return 1
        case "bar":
            return 2
        case "bar":    // a duplicate case: no errors/warnings are reported
            return 3
    }
}

Suggest A Solution

Validate and reject duplicate cases during compile time.