Fix a brace matching bug in Arena.py

Before, the range of valid values for scope and code positions overlapped, so,
naturally, some code ended up "tangled" with pointers going weird places. For
example, here's a Houndini program that escapes its seemingly inevitable death:

    .()*2<<<<<<<<<<((((-)*5>[(-)*4.[+.]].)*10000)*1)*1
Merge pull request #6 from Lymia/master

Fix a brace matching bug in Arena.py by Lymia