Attempt to fix codon Optimize rand usage

synthesis/codon/codon.go

@@ -156,10 +156,11 @@ func Optimize(aminoAcids string, codonTable Table, randomState ...int) (string,
 	}
 
 	// weightedRand library insisted setting seed like this. Not sure what environmental side effects exist.
+	var randomSource *rand.Rand
 	if len(randomState) > 0 {
-		rand.Seed(int64(randomState[0]))
+		randomSource = rand.New(rand.NewSource(int64(randomState[0])))
 	} else {
-		rand.Seed(time.Now().UTC().UnixNano())
+		randomSource = rand.New(rand.NewSource(time.Now().UTC().UnixNano()))
 	}
 
 	var codons strings.Builder
@@ -173,6 +174,7 @@ func Optimize(aminoAcids string, codonTable Table, randomState ...int) (string,
 		if !ok {
 			return "", invalidAminoAcidError{aminoAcid}
 		}
+		chooser.PickSource(randomSource)
 		codons.WriteString(chooser.Pick().(string))
 	}
 	return codons.String(), nil