Add SQL/92 "constants" current_date, current_time, and current_timestamp.

Add SQL/92 types decimal and numeric (temporary for syntax support).
 These types need more support in the backend to be really implemented,
 and the parser will need to be changed at that time.
Adjust limits on precision parameters for FLOAT(p) to match IEEE-compliant
 arithmetic. Perhaps these limits should be processor-specific or obtained
 from system include files instead.