#24 Switch completely to SRFI-69 hash-tables

[Tony Garnock-Jones]

The current SISC hash-table interface should be
switched to the SRFI-69 interface, and all uses of the
old interface should be updated to use the new one.
Perhaps a compatibility interface - (import
old-hashtable) - could be supplied, but (import
hashtable) should get SRFI-69.