Replace std.mem.copy with std.mem.copyForwards

[Rakhyvel]

A recent PR for the Zig standard library removes the std.mem.copy function, which was an alias for std.mem.copyForwards. This function was used in toOwned().

A proposed fix:

pub fn toOwned(self: String) Error!?[]u8 {
    if (self.buffer != null) {
        const string = self.str();
        if (self.allocator.alloc(u8, string.len)) |newStr| {
            std.mem.copyForwards(u8, newStr, string); // replace std.mem.copy with std.mem.copyForwards
            return newStr;
        } else |_| {
            return Error.OutOfMemory;
        }
    }

    return null;
}