Add keyExists(T) or hasKey(T) method for Array class?

[zoujiaqing]

code:

string[int] users;

users[10001] = Brian;
users[10002] = Heromyth

if (users.hasKey(10001))
{
    // OK
}

if (users.keyExists(10002))
{
    // OK
}

[Heromyth]

bool hasKey(K, V)(V[K] arr, K key) {
    auto v = key in arr;
    return v !is null;
}

unittest {
    string[int] arr;
    arr[1001] = "1001";
    arr[1002] = "1002";

    assert(arr.hasKey(1001));
    assert(!arr.hasKey(1003));
}