expr: remove --bignum and --no-bignum options

expr: remove --bignum and --no-bignum options

* doc/coreutils.texi (expr invocation): Remove the --bignum and
--no-bignum options.  They weren't really needed, and they broke
longstanding (albeit nonportable) scripts.
* src/expr.c: Don't include <assert.h>.  Include "inttostr.h",
"long-options.h", "verify.h".  Check at compile-time that
size_t fits in unsigned long int, as the code assumes this in
several places.
(HAVE_GMP): Define to 0 if not defined, for convenience.
(mpz_t, mpz_clear, mpz_init_set_ui, mpz_init_set_str, mpz_add):
(mpz_sub, mpz_mul, mpz_tdiv_q, mpz_tdiv_r, mpz_get_str, mpz_sgn):
(mpz_fits_ulong_p, mpz_get_ui, mpz_out_str):
Supply substitutes when !HAVE_GMP, which work well enough for
expr's purposes.
(mp_integer): Remove.  All integers are gmp, if gmp is available.
(struct valinfo): Remove 'z' member; no longer needed.  The 'i'
member is always of type mpz_t.
(enum arithmetic_mode, MP_NEVER, MP_ALWAYS, MP_AUTO, mode):
Remove; no longer needed.
(usage): Remove documentation of --bignum and --no-bignum.
(integer_overflow): Abort if error misbehaves, to pacify GCC.
Restore old message on arithmetic overflow, to be conservative.
(die): Omit exit_status parameter; not needed (is always EXPR_FAILURE).
(string_too_long, USE_BIGNUM, NO_USE_BIGNUM, long_options):
Remove; no longer needed.
(main): Don't use getopt_long; this breaks old nonportable scripts.
(int_value): Arg is unsigned, in case we have strings whose length
exceeds LONG_MAX (!).
(int_value, freev, printv, null, tostring, toarith):
(eval6, eval4, eval3):
Always use mpz_ functions, to simplify the code.
(substr_value): Remove; no longer needed.
(getsize): Simplify the API: one arg rather than 3.  Don't assume
unsigned long int fits in size_t.
(promote, domult, dodivide, doadd): Remove; no longer needed.
* tests/misc/expr: Don't use --bignum to test for bignum support.
Instead, use big numbers to test this.