This adds support for setting the new payment metadata field in
BOLT11 invoices, using a new type flag on the builder to enforce
transition correctness.
We allow users to set the payment metadata as either optional or
required, defaulting to optional so that invoice parsing does not
fail if the sender does not support payment metadata fields.
This matches the spec and helps avoid any confusion around
naming. We're also then consistent with `cltv_expiry` in an HTLC being
the actual block height value for the CLTV and not a delta.
The BOLT 11 invalid invoice test vectors suggest failing to parse
invoices which have an amount which is not a whole number of
millisatoshis. lightning-invoice, however, happily parses such
invoices. While we could continue to parse them, failing them makes
for one less check on the user code side, so we might as well.
In order to keep the invoice creation less likely to fail, we also
switch the Builder amount-setting function to use millisatoshis.
This adds two additional tests from the BOLT 11 invalid invoice
tests, fixing the two errors that broke them. It fixes a panic on
the "nonrecoverable signature" test and makes the error variant
more sensible on the bogus SI prefix test.
Instead of relying on users to set an invoice's features correctly,
enforce the semantics inside InvoiceBuilder. For instance, if the user
sets a PaymentSecret then InvoiceBuilder should ensure the appropriate
feature bits are set. Thus, for this example, the TaggedField
abstraction can be retained while still ensuring BOLT 11 semantics at
the builder abstraction.
