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.

CHANGELOG.md

@@ -6,6 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- Add `descriptor::checksum::get_checksum_bytes` method.
 
 ## [v0.20.0] - [v0.19.0]