zkdsl compiler: remove the possibility for a range loop to mutate variables defined in external scope. Use manual buffers instead.

[TomWambsgans]

Before:

total: Mut = 0
for i in range(0, 11):
    total += i 
assert total == 55

After:

total_buf = Array(11) 
total_buf[0] = 0
for i in range(0, 11):
    v: Mut = total_buf[i]
    v += i
    total_buf[i + 1] = v
total = total_buf[10]
assert total == 55