solana: Change transceiver guard to anchor constraint

Adds methods on the repo's custom Bitmap struct to expose length-related
methods from the underlying Bitmap struct. This allows the Anchor code
to check the length of the Bitmap which corresponds to the number of
enabled transceivers.

Author: johnsaigle

solana/programs/example-native-token-transfers/src/bitmap.rs

@@ -35,6 +35,14 @@ impl Bitmap {
         let bm = BM::<128>::from_value(self.map) & BM::<128>::from_value(enabled.map);
         bm.len() as u8
     }
+
+    pub fn len(self) -> usize {
+        BM::<128>::from_value(self.map).len()
+    }
+
+    pub fn is_empty(self) -> bool {
+        BM::<128>::from_value(self.map).is_empty()
+    }
 }
 
 #[cfg(test)]

solana/programs/example-native-token-transfers/src/instructions/transfer.rs

@@ -22,6 +22,11 @@ pub struct Transfer<'info> {
     #[account(mut)]
     pub payer: Signer<'info>,
 
+    // Ensure that there exists at least one enabled transceiver
+    #[account(
+        constraint = config.next_transceiver_id != 0 @ NTTError::NoRegisteredTransceivers,
+        constraint = config.enabled_transceivers.is_empty() @ NTTError::NoRegisteredTransceivers,
+    )]
     pub config: NotPausedConfig<'info>,
 
     #[account(
@@ -121,14 +126,6 @@ pub fn transfer_burn(ctx: Context<TransferBurn>, args: TransferArgs) -> Result<(
 
     let accs = ctx.accounts;
 
-    // TODO If functionality is added to disable transceivers, this check should be modified to
-    // ensure that, if all transceivers have been disabled, that an error is returned. Otherwise it
-    // may be possible to permanently lose funds.
-    // No transceivers have been registered yet
-    if accs.common.config.next_transceiver_id == 0 {
-        return Err(NTTError::NoRegisteredTransceivers.into());
-    }
-
     let TransferArgs {
         mut amount,
         recipient_chain,
@@ -223,14 +220,6 @@ pub fn transfer_lock(ctx: Context<TransferLock>, args: TransferArgs) -> Result<(
 
     let accs = ctx.accounts;
 
-    // TODO If functionality is added to disable transceivers, this check should be modified to
-    // ensure that, if all transceivers have been disabled, that an error is returned. Otherwise it
-    // may be possible to permanently lose funds.
-    // No transceivers have been registered yet
-    if accs.common.config.next_transceiver_id == 0 {
-        return Err(NTTError::NoRegisteredTransceivers.into());
-    }
-
     let TransferArgs {
         mut amount,
         recipient_chain,