Add k-mer encoding and decoding functions with normalized k-mer support

This commit introduces new functions for encoding and decoding k-mers, including support for normalized k-mers. It also updates the frequency filter and k-mer set implementations to use the new encoding functions, providing zero-allocation encoding for better performance. The commit hash has been updated to reflect the latest changes.

pkg/obioptions/version.go

@@ -8,7 +8,7 @@ import (
 // corresponds to the last commit, and not the one when the file will be
 // commited
 
-var _Commit = "afcb43b"
+var _Commit = "c5dd477"
 var _Version = "Release 4.4.0"
 
 // Version returns the version of the obitools package.