Faster compilation speed

[Jamie Lokier]

Timothy J. Wood wrote:
> 		// This function adds an entry of 1 to the external hash table if there
> 		// was no entry, otherwise it increments the existing entry.
> 		IncrementExternalRC(this);

Nice idea, although you might be able to amortise the hash table
accesses some more by storing multiples of 64 in the external hash
table.  When the internal count overflows, transfer 64 counts to the
hash table, and vice versa when it reaches zero.

-- Jamie