Reduce the memory requirement for large ispell dictionaries.

This patch eliminates per-chunk palloc overhead for most small allocations
needed in the representation of an ispell dictionary.  This saves close to
a factor of 2 on the current Czech ispell data.  While it doesn't cover
every last small allocation in the ispell code, we are at the point of
diminishing returns, because about 95% of the allocations are covered
already.

Pavel Stehule, rather heavily revised by Tom