Refactor splits/2 using foreach syntax (remove private filter _nwise) (#3260)

This commit refactors `splits/2` implementation using `foreach` syntax
and reduces intermediate arrays. This refactoring removes an unnecessary
private (and undocumented) filter `_nwise` and closes #3148.

tests/onig.test

@@ -192,8 +192,20 @@ sub("(?<x>.)"; "\(.x)!")
 "aB"
 ["AB","ab","cc"]
 
-# splits and _nwise
+# splits
 [splits("")]
 "ab"
 ["","a","b",""]
 
+[splits("c")]
+"ab"
+["ab"]
+
+[splits("a+"; "i")]
+"abAABBabA"
+["","b","BB","b",""]
+
+[splits("b+"; "i")]
+"abAABBabA"
+["a","AA","a","A"]
+