Transition to an instance-based API and introduce deletion methods

The new instance system allows the package to manage as many
databases as necessary simultaneously by storing database handles
within each instance object rather than within a global variable.
Not only does this extend the capabilities of the package, but it
also contributes to ensuring that the database is correctly
prepared before any other methods depending upon its structure are
called.

Besides this major change, the new methods `Delete` and
`MustDelete` have been introduced to handle deletion.