Introduce get_checksum_bytes method and improvements

`get_checksum_bytes` returns a descriptor checksum as `[u8; 8]` instead
of `String`, potentially improving performance and memory usage.

In addition to this, since descriptors only use charaters that fit
within a UTF-8 8-bit code unit, there is no need to use the `char` type
(which is 4 bytes). This can also potentially bring in some performance
and memory-usage benefits.

src/descriptor/error.rs

@@ -28,8 +28,8 @@ pub enum Error {
     /// Error while extracting and manipulating policies
     Policy(crate::descriptor::policy::PolicyError),
 
-    /// Invalid character found in the descriptor checksum
-    InvalidDescriptorCharacter(char),
+    /// Invalid byte found in the descriptor checksum
+    InvalidDescriptorCharacter(u8),
 
     /// BIP32 error
     Bip32(bitcoin::util::bip32::Error),