The size of the small string optimization was hard coded. For some use
cases you want a bigger or smaller size. It is now configurable by a
template parameter. For that we changed the name to BasicSmallString and
added an alias of BasicSmallString<31> to SmallString.

Change-Id: I844b4420d260290307a6018bb6cc4cf3ba7bd449
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>