Use Infallible for the unconstructable default custom message type

Use Infallible for the unconstructable default custom message type

When we landed custom messages, we used the empty tuple for the
custom message type for `IgnoringMessageHandler`. This was fine,
except that we also implemented `Writeable` to panic when writing
a `()`. Later, we added support for anchor output construction in
CommitmentTransaction, signified by setting a field to `Some(())`,
which is serialized as-is.

This causes us to panic when writing a `CommitmentTransaction`
with `opt_anchors` set. Note that we never set it inside of LDK,
but downstream users may.

Instead, we implement `Writeable` to write nothing for `()` and use
`core::convert::Infallible` for the default custom message type as
it is, appropriately, unconstructable.

This also makes it easier to implement various things in bindings,
as we can always assume `Infallible`-conversion logic is
unreachable.