collapseos/blk/076

STACK OVERFLOW PROTECTION: To avoid having to check for stack
underflow after each pop operation (which can end up being
prohibitive in terms of costs), PS_ADDR should be set to
at least 6 bytes before its actual limit. 6 bytes because we
seldom have words requiring more than 3 items from the stack.
Then, at each "exit" call we check for stack underflow.