expr: support arbitrary-precision arithmetic

* src/Makefile.am (expr_LDADD): Link expr against GNU MP.
* doc/coreutils.texi (expr invocation): Describe --bignum,
--no-bignum.   Explain the new arbitrary-precision functionality.
* NEWS: Indicate that arbitrary-precision arithmetic is now
supported in expr.
* src/expr.c (enum valtype): Added mp_integer, signifying a GNU MP
number.
(usage): Document the new options --bignum and --no-bignum which
force and prohibit the use of arbitrary-precision arithmetic,
respectively.
(long_options): data structure for getopt_long, which we need to
use to parse the options mentioned above.
(main): parse these options with getopt_long instead of
parse_long_options.
(valinfo): Downgrade the numeric member of the union from
intmax_t to signed long, since MP lacks functions for promoting an
intmax_t to an arbitrary-precision quantity.
(enum arithmetic_mode): Represents the current choice between
--bignum, --no-bignum and the default (automatically switch from
one to the other if needed).
(integer_overflow): issue a more explicit error message indicating
that MP is not available.
(string_too_long): new function, emits a fatal error message for
the case where an argument to the 'index' expression is too long
for a string offset to be represented.
(int_value): With --bignum, create the value as mp_integer rather
than plain integer.
(substr_value): factored out of eval6; implements "substr".
(freev): also destroy mp_integer values.  Check that no mp_integer
values exist if --no-bignum was specified.
(printv, null, tostring): support mp_integer.
(toint): new funtion for converting from string or mp_integer to
integer.
(getsize): extracts a size_t value from a VALUE object; used to
implement substr.
(promote): promotes a value from integer to mp_integer.
(domult, dodivide): functions for multiplication and division,
factored out of eval4.
(doadd): addition/subraction function, factpred out of eval3.
(eval3): support mp_integer types; call doadd.
(eval4): support mp_integer types; call domult, dodivide.
(eval6): support mp_integer offsets and lengths for "substr" and
"index".
* TODO: Mention that expr supports arbitrary-precision arithmetic,
and suggest that this might also be a good idea for seq.
* AUTHORS (expr): Add James Youngman.

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