This commit consolidates `DefLocal` and `DefGlobal` into a single
`Define` bound kind. This simplifies the AST and makes it more
consistent.
It also introduces `DeclarationKind` to differentiate between variable
and parameter definitions.
The `flatten_tuple` function was unnecessarily recursive. It can now
simply return the elements of the tuple directly. The VM's destructuring
logic has been updated to handle nested destructuring more efficiently.
A new integration test case for multi-level destructuring has been
added.