Add `operator+` overload for `T` and `Vector<T>`

[htiek]

Currently, you can write vec + elem to get back a new Vector that consists of all the elements of vec with elem appended to the. end. However, the reverse elem + vec does not compile. We should consider adding an overload of operator+ that addresses this.

[zelenski]

vec + elem same as elem + vec ? The latter seems to show intent to prepend element rather than append. Do you see a significant use case for it?

I am generally a fan of reduce/maintain smaller library footprint, esp. for interfaces we publish to students...

[htiek]

I agree that a smaller interface is a nice thing to have.

There's a nice parallel between the overloads for Vector<T> and for std::string. You can write str + ch to form a new string with a character at the end, and you can write ch + str to form a new string with a character added at the beginning. I had some students ask me a while back why the same doesn't work for Vector. This particularly came up with some recursion warm-up exercises where code for strings looks really different than code for Vectors. An example:

/* Totally fine */
string toLowerCase(const string& input) {
    if (input == "") return "";
    return tolower(input[0]) + toLowerCase(input.substr(1));
}

/* Oops, doesn't compile. */
Vector<char> toLowerCase(const Vector<char>& input) {
    if (input.isEmpty()) return { };
    return tolower(input[0]) + toLowerCase(input.subList(1));
}