Replace '__gmp_const' with 'const'

The __gmp_const macro was added to GMP as a workaround for C compilers
which didn't support the const keyword.  GMP 5.1 removed support for
pre-ANSI C compilers, so the __gmp_const workaround was removed at the
same time.

This change replaces all uses of '__gmp_const' with uses of the 'const'
keyword directly, since MLton already expects C99 support from the system
C compiler.

This should fix the build errors for systems using GMP >= 5.1, as reported in https://github.com/MLton/mlton/issues/3.

Tested on amd64-freebsd (with GMP 5.1.1) and x86-linux (with GMP 5.0.2).