Make string mutable

[Jiyang (Mark) Tang]

Code like this doesn't work:

pub fn main() : int {
    var s = "string";
    s[0] = 'p';
    assert(s[0] == 'p');
    return 0;
}

[Jiyang (Mark) Tang]

After reconsidering the language design, I decided to make builtin str immutable and be stored on stack. A container in std will contain an implementation of a growable and mutable string.