Modify 'expr' so that it uses intmax_t, not int, to calculate

Modify 'expr' so that it uses intmax_t, not int, to calculate
the value of integer expressions.

(struct valinfo.i): Now intmax_t, not int.
(docolon, int_value, str_value, isstring, nextarg, toarith,
freev, tostring, trace): Remove unnecessary forward decls.
(int_value, printv, tostring, toarith, arithf, arithdivf, docolon,
eval6, eval4, eval3): Do integer arithmetic using intmax_t, not int.
(docolon): Don't assume size_t fits in int.