Use brace counter to keep track of matching braces in BF implementation

Based on C++ reference implementation of dbfi (see http://www.hevanet.com/cristofd/08.html)

Doing it in Hugo is ugly, but in pseudo-Python, it's fairly easy enough to follow:

if pc == '[' and not memc:
  braces = 1
  while braces:
    pc += 1
    if pc == '[':
      braces += 1
    elif pc == ']':
      braces -= 1

Situation is roughly symmetrical for ']' - just flip increments, decrements.