Several optimizations that came from KCacheGrind / calltree fun.

Several optimizations that came from KCacheGrind / calltree fun.
Basically they fall into the categories:
- Don't convert things before you need to
- When you do, use more effecient copy constructors / assignment operators
  (i.e. when copying prefer memcpy to a for loop and when that's not possible
  use an iterator instead of operator[])

git-svn-id: svn://anonsvn.kde.org/home/kde/trunk/kdesupport/taglib@301896 283d02a7-25f6-0310-bc7c-ecb5cbfe19da