Make sqlite3_stmt.db public

[i-sinister]

Currently sqlite3_stmt.db is declared as internal however using sqlite3_stmt requires sqlite3 instance to be available for error core checking after sqlite3_step, sqlite3_bind_XXX or sqlite3_reset calls. So in the application sqlite3 and sqlite3_stmt have to be used together. As sqlite3_stmt already has a reference to sqlite3 making it public will make coding experience better and "improve performance" by eliminating the need of tossing second sqlite3 pointer around.