Un-cfg-gate StaticInvoices

We need to include static invoices in the public API as part of the onion
messages we're adding for static invoice server support. Utilities to create
these static invoices and other parts of the async receive API will remain
cfg-gated for now. Generally, we can't totally avoid exposing half baked async
receive support in the public API because OnionMessenger is parameterized by an
async payments message handler, which can't be cfg-gated easily.

This contains the changes to the public API, to the original commit
made by Val, to fix the following clippy warning:

```
error: methods called `from_*` usually take no `self`
   --> lightning/src/offers/static_invoice.rs:393:32
    |
393 |     pub(crate) fn from_same_offer(&self, invreq: &InvoiceRequest) -> bool {
    |                                   ^^^^^
    |
    = help: consider choosing a less ambiguous name
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#wrong_self_convention
    = note: `-D clippy::wrong-self-convention` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::wrong_self_convention)]`

error: could not compile `lightning` (lib) due to 1 previous error
```

```
error[E0599]: no method named `bytes` found for reference `&InvoiceRequest` in the current scope
   --> lightning/src/offers/static_invoice.rs:387:34
    |
387 |             Offer::tlv_stream_iter(invreq.bytes()).map(|tlv_record| tlv_record.record_bytes);
    |                                           ^^^^^-- help: remove the arguments
    |                                           |
    |                                           field, not a method
    |
    = help: items from traits can only be used if the trait is implemented and in scope
    = note: the following trait defines an item `bytes`, perhaps you need to implement it:
            candidate #1: `std::io::Read`

warning: variable does not need to be mutable
   --> lightning/src/offers/offer.rs:261:4
    |
261 |             mut $self: $self_type, metadata: Vec<u8>,
    |             ----^^^^^
    |             |
    |             help: remove this `mut`
...
534 |     offer_explicit_metadata_builder_methods!(self, &mut Self, (), ());
    |     ----------------------------------------------------------------- in this macro invocation
    |
    = note: `#[warn(unused_mut)]` on by default
    = note: this warning originates in the macro `offer_explicit_metadata_builder_methods` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0599`.
```

Signed-off-by: Vincenzo Palazzo <[email protected]>

Author: valentinewallace

lightning/src/ln/outbound_payment.rs

@@ -1104,7 +1104,7 @@ impl OutboundPayments {
 						.as_ref()
 						.ok_or(Bolt12PaymentError::UnexpectedInvoice)?
 						.invoice_request;
-					if !invoice.from_same_offer(invreq) {
+					if !invoice.build_from_same_offer(invreq) {
 						return Err(Bolt12PaymentError::UnexpectedInvoice)
 					}
 					if invoice.invoice_features().requires_unknown_bits_from(&features) {

lightning/src/offers/invoice_request.rs

@@ -858,7 +858,7 @@ impl InvoiceRequest {
 	);
 	invoice_request_verify_method!(self, Self);
 
-	#[cfg(async_payments)]
+	#[allow(unused)] // TODO: remove this once we remove the `async_payments` cfg flag
 	pub(super) fn bytes(&self) -> &Vec<u8> {
 		&self.bytes
 	}
@@ -874,6 +874,11 @@ impl InvoiceRequest {
 		InvoiceWithExplicitSigningPubkeyBuilder
 	);
 	invoice_request_verify_method!(self, &Self);
+
+	#[allow(unused)] // TODO: remove this once we remove the `async_payments` cfg flag
+	pub(super) fn bytes(&self) -> &Vec<u8> {
+		&self.bytes
+	}
 }
 
 impl InvoiceRequest {

lightning/src/offers/mod.rs

@@ -25,7 +25,6 @@ pub mod parse;
 mod payer;
 pub mod refund;
 pub(crate) mod signer;
-#[cfg(async_payments)]
 pub mod static_invoice;
 #[cfg(test)]
 pub(crate) mod test_utils;

lightning/src/offers/offer.rs

@@ -743,7 +743,6 @@ impl Offer {
 			.chain(TlvStream::new(bytes).range(EXPERIMENTAL_OFFER_TYPES))
 	}
 
-	#[cfg(async_payments)]
 	pub(super) fn verify<T: secp256k1::Signing>(
 		&self, nonce: Nonce, key: &ExpandedKey, secp_ctx: &Secp256k1<T>,
 	) -> Result<(OfferId, Option<Keypair>), ()> {

lightning/src/offers/static_invoice.rs

@@ -379,7 +379,8 @@ impl StaticInvoice {
 		self.signature
 	}
 
-	pub(crate) fn from_same_offer(&self, invreq: &InvoiceRequest) -> bool {
+	#[allow(unused)] // TODO: remove this once we remove the `async_payments` cfg flag
+	pub(crate) fn build_from_same_offer(&self, invreq: &InvoiceRequest) -> bool {
 		let invoice_offer_tlv_stream =
 			Offer::tlv_stream_iter(&self.bytes).map(|tlv_record| tlv_record.record_bytes);
 		let invreq_offer_tlv_stream =