Make Str, Int and Num all immutable

[stevan]

This falls under the "what would Perl 6 do" umbrella. This means removing the increment operators (people can use regular old addition instead), and making Str.chop and Str.chomp return new strings rather then alter in place. This will remove all usage of MoeNativeObject.setNativeValue except for Array.bind_pos, which uses it to autogrow the array size (should look in the docs and see if this can be done another way).