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. the `sqlite3::ffi` module has been also audited and
revised to use mutable raw pointers whenever appropriate.
Merge pull request #86 from lifthrasiir/const-ptr

Language changes: *T -> *const T.