New system function TRUNCATE_VALUE to truncate a value to the required precision.

h2/src/test/org/h2/test/test-1.3.txt

 --- special grammar and test cases ---------------------------------------------------------------------------------------------
-call cast_precision('Test 123', 4, false);
+call truncate_value('Test 123', 4, false);
 > 'Test'
 > ------
 > Test
 > rows: 1
 
-call cast_precision(1234567890.123456789, 4, false);
+call truncate_value(1234567890.123456789, 4, false);
 > exception
 
-call cast_precision(1234567890.123456789, 4, true);
+call truncate_value(1234567890.123456789, 4, true);
 > 1234567890.1234567
 > ------------------
 > 1234567890.1234567