Refactor the main parser to use the `bind` macro.

The metabang-bind lib offers a nice bind macro that solves the problem
of ignoring bindings in destructuring-bind, and allows a let* approach
to nested destructuring (wven when mixed with let declarations).

Using that lib (that we already indirectly depend on anyway) simplifies
the parser code substantially.
Register the new dependency towards cl-metabang-bind in the debian package.