Eliminating unnecessary speller creation/deletion.

Previously, an internal speller object was created and destroyed for every call to #correct? and #suggestions. Now, an internal speller object is only allocated when the initial FFI::Aspell::Speller object is created and when its configuration changes.

To cleanup native speller objects, we leverage Ruby's finalizers. In the FFI::Aspell::Speller finalizer, we free the current FFI speller object, allowing us to maintain API compatibility while still benefitting from the performance improvement.
Ignoring generated YARD doc directory.