[str] Optimized Parrot_str_concat() to avoid making a COW STRING of the wrong size, then having to resize it immediately.  Creating a new STRING of the proper size runs the GC a lot less.
[str] Optimized Parrot_str_concat() to avoid making a COW STRING of the wrong
size, then having to resize it immediately.  Creating a new STRING of the
proper size runs the GC a lot less.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@45367 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

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