IdnMapping unnecessarily allocates substrings

[jamesqo]

Discovered this while going through the code in dotnet/corefx#7291. It looks like IdnMapping is allocating a substring where it shouldn't need to, specifically here and here. I'm going to open up a PR in a couple of days for this when I have time, but I'm raising an issue here first as a self-reminder.

[steveharter]

@jamesqo The code in question allocates a substring like:

unicode = unicode.Substring(index, count);

If index !=0 and count != unicode.Length then the allocation isn't necessary, otherwise it is. I assume the intended optimization is:

if (index != 0 || count != unicode.Length)
unicode = unicode.Substring(index, count);

[svick]

@steveharter string.Substring() is optimized for that and already doesn't allocate a new string in that case.