Allow CAST expressions and unary "+" operators to be used in the DEFAULT

argument of an ALTER TABLE ADD COLUMN and to be understand on the RHS of
range constraints interpreted by STAT3/4.  This involves a rewrite of the
implementation of the CAST operator.

FossilOrigin-Name: 91d8a8d0b792ea5c4fe68fd9caaf3345eddea486

test/alter4.test

@@ -145,7 +145,7 @@ do_test alter4-2.6 {
 } {1 {Cannot add a column with non-constant default}}
 do_test alter4-2.7 {
   catchsql {
-    alter table t1 add column d default (-+1);
+    alter table t1 add column d default (-5+1);
   }
 } {1 {Cannot add a column with non-constant default}}
 do_test alter4-2.99 {