Our current expression API has a `to_null_threshold` method, which works
reasonably well but requires the caller translate the returned threshold
by accessing individual children.

We will later want to change the Tree represntation to make individual
child access inefficient. To do this, encapsulate the threshold
construction into a new verify_threshold.

This previously was not done because our messy error structures made it
very difficult to manage. But our error structures are less messy so
it's possible now, though a bit of a hack. (We map everything to the
global Error structure. Later we will do pretty-much the same thing
but with ParseError in place of Error, which will be more elegant.)

This removes the ability to randomly access children of tree nodes. You
can get them in order using the `children` iterator, and get them
recursively using the `TreeLike` iterator methods. (In the next commits
we will specialize these a bit, providing a `pre_order_iter` and
`rtl_post_order_iter` which let you efficiently skip over subtrees. We
will need these to parse Taproot expression trees and they don't fit
into the `TreeLike` trait, at least not efficiently.)

This significantly speeds up and simplifies tree parsing, at the cost of
having a more complicated API (but we mostly addressed the API question
in the previous commits).

This completely eliminates recursion for the Tree data type, including
in the Drop impl.

Big diff but there are only two "real" changes -- expression/mod.rs is
substantially rewritten of course since we replace the core datatype,
and Tr::from_tree is substantially rewritten since doing so was the
point of this change. The rest of the changes are mechanically changing
the signature of expression::FromTree::from_tree everywhere.

This bumps the local Cargo.toml version to 13, which will be the next
release (since we've made many breaking changes), and in the fuzz test
adds an explicit dependency on miniscript 12 from crates.io, as
`old_miniscript`.

Adds a single fuzztest which attempt to parse descriptors with both
master and 12, to make sure they're the same.