expr: fix comparisons 'a < b' where we were overflowing a-b

expr: fix comparisons 'a < b' where we were overflowing a-b
(not to mention that we used int, not arith_t). closes bug 2744.
Also, shrink a bit and add testsuite entry

function                                             old     new   delta
nextarg                                               75      84      +9
tostring                                              38      35      -3
toarith                                               89      86      -3
str_value                                             35      32      -3
eval6                                                555     552      -3
int_value                                             29      23      -6
eval4                                                128     120      -8
eval3                                                112     104      -8
eval2                                                512     417     -95
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(add/remove: 0/0 grow/shrink: 1/8 up/down: 9/-129)           Total: -120 bytes