Use bech32 checksum module

Currently we have a custom implementation for the BCH codes used in
calculating the desciptor checksum. We recently released an
implementation in `bech32` of the same thing, we can use it here.

Encoding descriptors with a checksum differs from bech32 addresses
because of the `#` symbol that separates the descriptor from the
checksum so we cannot use the iterators from `bech32`, instead we define
our own iterator that adds the checksum and yields `chars`.

Cargo.toml

@@ -12,8 +12,8 @@ edition = "2018"
 
 [features]
 default = ["std"]
-std = ["bitcoin/std", "bitcoin/secp-recovery"]
-no-std = ["bitcoin/no-std"]
+std = ["bitcoin/std", "bitcoin/secp-recovery", "bech32/std"]
+no-std = ["bitcoin/no-std", "bech32/alloc"]
 compiler = []
 trace = []
 
@@ -22,6 +22,7 @@ rand = ["bitcoin/rand"]
 base64 = ["bitcoin/base64"]
 
 [dependencies]
+bech32 = { version = "0.10.0-alpha", default-features = false }
 bitcoin = { version = "0.30.0", default-features = false }
 internals = { package = "bitcoin-private", version = "0.1.0", default_features = false }
 

src/descriptor/bare.rs

@@ -7,12 +7,12 @@
 //! Also includes pk, and pkh descriptors
 //!
 
-use core::fmt;
+use core::fmt::{self, Write as _};
 
 use bitcoin::script::{self, PushBytes};
 use bitcoin::{Address, Network, ScriptBuf};
 
-use super::checksum::{self, verify_checksum};
+use super::checksum::{verify_checksum, Checksummed};
 use crate::descriptor::DefiniteDescriptorKey;
 use crate::expression::{self, FromTree};
 use crate::miniscript::context::{ScriptContext, ScriptContextError};
@@ -157,10 +157,17 @@ impl<Pk: MiniscriptKey> fmt::Debug for Bare<Pk> {
 
 impl<Pk: MiniscriptKey> fmt::Display for Bare<Pk> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        use fmt::Write;
-        let mut wrapped_f = checksum::Formatter::new(f);
-        write!(wrapped_f, "{}", self.ms)?;
-        wrapped_f.write_checksum_if_not_alt()
+        let s = format!("{}", self.ms);
+
+        if f.alternate() {
+            write!(f, "{}", s)?;
+        } else {
+            for ch in Checksummed::new_unchecked(&s) {
+                f.write_char(ch)?;
+            }
+        }
+
+        Ok(())
     }
 }
 
@@ -354,10 +361,17 @@ impl<Pk: MiniscriptKey> fmt::Debug for Pkh<Pk> {
 
 impl<Pk: MiniscriptKey> fmt::Display for Pkh<Pk> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        use fmt::Write;
-        let mut wrapped_f = checksum::Formatter::new(f);
-        write!(wrapped_f, "pkh({})", self.pk)?;
-        wrapped_f.write_checksum_if_not_alt()
+        let s = format!("pkh({})", self.pk);
+
+        if f.alternate() {
+            write!(f, "{}", s)?;
+        } else {
+            for ch in Checksummed::new_unchecked(&s) {
+                f.write_char(ch)?;
+            }
+        }
+
+        Ok(())
     }
 }