A BOLT 12 test vector uses an `invoice_request` message that has a
currency, which aren't supported, so using OfferBuilder::build_unchecked
is required to avoid a panic.
Add a builder for creating invoice requests for an offer given a
payer_id. Other settings may be optional depending on the offer and
duplicative settings will override previous settings. Building produces
a semantically valid `invoice_request` message for the offer, which then
can be signed for the payer_id.
When reading an offer, an `invoice_request` message is sent over the
wire. Implement Writeable for encoding the message and TryFrom for
decoding it by defining in terms of TLV streams. These streams represent
content for the payer metadata (0), reflected `offer` (1-79),
`invoice_request` (80-159), and signature (240).
Offers uses a merkle root hash construction for signature calculation
and verification. Add a submodule implementing this so that it can be
used when parsing and signing invoice_request and invoice messages.
