Language changes: *T -> *const T.

This commit also fixes various places where an immutable value is abused as a mutable argument to the SQLite function, which can be very unsafe and prone to otherwise safe optimizations. (An [example](https://github.com/linuxfood/rustsqlite/blob/05d93c1/src/sqlite3/database.rs#L72-L83): `new_stmt` is immutable but it gets changed!) The `sqlite3::ffi` module has been also audited and revised to use mutable raw pointers whenever appropriate.