This change adds a little breaking-change in the schema and the codegen but fixes a problem with GoType.

After this change, users that use a GoType(&T{}) will get *T in all generated interfaces (instead of T). However, if users still want to use values (instead of pointers), they can pass T{} to GoType (e.g. GoType(T{})), even if T doesn't implement the sql.Scanner interface (but *T does).

Also, this change now generates the basic predicates to driver.Valuer types (e.g. a struct used in GoType). The basic predicates are: =, <>, <, <=, >, =>, IN, NOT IN. For optional fields, it generates also the IS NULL and IS NOT NULL predicates.