(toarith): Rewrite to detect/diagnose integer overflow,

(toarith): Rewrite to detect/diagnose integer overflow,
rather than suffering silently.
Before, expr would silently overflow and wrap around:
  $ expr 9223372036854775808 = 0   # $(echo 2^63|bc)
  1
Now it detects the problem and exits nonzero:
  $ ./expr $(echo 2^63|bc) = 0
  ./expr: 9223372036854775808: integer is too large