Support default arguments in function definitions

[msakuta]

This feature exists in C++, Python and many other languages (but interestingly not in Rust).

You can define a function like this:

fn add(a: i32 = 1, b: i32 = 2) -> i32 {
    a + b;
}

and you can call this function without explicit arguments:

print(add());

and it should print 3.

You can also partially provide an argument:

print(add(1000));

which should print 1002.

See the wiki for more details.