Bindings can't handle references in return types, so reduce the
visibility to pub(crate).

The bindings currently get confused by the implicit `Sign` type, so
we temporarily remove it on the `impl` here.

Re-exports in Rust make `use` statements a little shorter, but for
otherwise don't materially change a crate's API. Sadly, the C
bindings generator currently can't figure out re-exports, but it
also exports everything into one global namespace, so it doesn't
matter much anyway.

Having struct fields with references to other structs is tough in
our bindings logic, but even worse if the fields are in an enum.
Its simplest to just take the clone penalty here.

The scorer currently relies on an associated type for the fee
parameters. This isn't supportable at all in bindings, and for
lack of a better option we simply hard-code the parameter for all
scorers  to `ProbabilisticScoringFeeParameters`.

These are not expressible in C/most languages, and thus must be
hidden.

Rather than `ChannelMonitor` only being clonable when the signer is
clonable, we require all signers to be clonable and then make all
`ChannelMonitor`s clonable.

…ructs

As we cannot express slices without inner references in bindings
wrappers.

Bindings don't accept dyn traits, but instead map any traits to a
single dynamic struct. Thus, we can always take a specific trait
to accept any implementation, which we do here.

…o map

Mapping an `Hmac<Sha256>` would require somewhat custom logic as
we'd have to behave differently based on generic parameters, so its
simplest to just swap it to a `[u8; 32]` instead.

The bindings really should support this, but currently they don't
and its late enough in the release cycle I don't want to try to fix
that.