Replace SplitInTwo with LeftSplitInTwo/RightSplitInTwo for precise splitting

Replace SplitInTwo calls with LeftSplitInTwo or RightSplitInTwo depending on the intended split direction. In fastseq_json_header.go, extract rank from suffix without splitting; in biosequenceslice.go and taxid.go, use LeftSplitInTwo to split from the left; add RightSplitInTwo utility function for splitting from the right.

pkg/obiutils/strings.go

@@ -144,7 +144,7 @@ func (r *AsciiSet) TrimLeft(s string) string {
 	return s[i:]
 }
 
-func SplitInTwo(s string, sep byte) (string, string) {
+func LeftSplitInTwo(s string, sep byte) (string, string) {
 	i := 0
 	for ; i < len(s); i++ {
 		c := s[i]
@@ -157,3 +157,17 @@ func SplitInTwo(s string, sep byte) (string, string) {
 	}
 	return s[:i], s[i+1:]
 }
+
+func RightSplitInTwo(s string, sep byte) (string, string) {
+	i := len(s) - 1
+	for ; i >= 0; i-- {
+		c := s[i]
+		if c == sep {
+			break
+		}
+	}
+	if i == len(s) {
+		return s, ""
+	}
+	return s[:i], s[i+1:]
+}