Add `Alphabet::as_str` method to get all symbols from an alphabet

lightmotif/src/abc.rs

@@ -56,6 +56,9 @@ pub trait Alphabet: Debug + Copy + Default + 'static {
 
     /// Get all the symbols of this alphabet.
     fn symbols() -> &'static [Self::Symbol];
+
+    /// Get a string with all symbols from this alphabet.
+    fn as_str() -> &'static str;
 }
 
 // --- ComplementableAlphabet --------------------------------------------------
@@ -94,6 +97,10 @@ impl Alphabet for Dna {
             Nucleotide::N,
         ]
     }
+
+    fn as_str() -> &'static str {
+        b"ACTGN"
+    }
 }
 
 /// A deoxyribonucleotide.
@@ -201,6 +208,10 @@ impl Alphabet for Protein {
             AminoAcid::X,
         ]
     }
+
+    fn as_str() -> &'static str {
+        "ACDEFGHIKLMNPQRSTVWYX"
+    }
 }
 
 /// A proteinogenic amino acid.