Owned and ref versions of Bolt11InvoiceDescription

Owned and ref versions of Bolt11InvoiceDescription

Split Bolt11InvoiceDescription into a version used with references to
the description or description hash in the invoice and an owned version
of these for when constructing an invoice. The latter is useful as it
removes an unnecessary clone and can be used in a future change
specifying either a description or description hash in larger set of
invoice parameters. Since it doesn't use a reference, it can be exposed
in bindings as well.