Closes Bears-R-Us#2886: Cap small strs groupby optimization

This PR (closes Bears-R-Us#2886) caps strs for the small str optimization at 8 bytes since there was a drop off in our str groupby benchmarks

This is because we currently have our small str optimization set to kick in at 16 bytes (which is how long the strings are in the multi-col str groupby benchmark) but this is the max number of bits we can have before we hit the totalDigits > 8 case, where we hash everything anyway. So these are all resulting in extra processing just to do what we were already doing

src/AryUtil.chpl

@@ -235,7 +235,7 @@ module AryUtil
       var allSmallStrs = true;
       var extraArraysNeeded = 0;
       var numStrings = 0;
-      const smallStrCap = 17;  // one bigger to ignore null byte
+      const smallStrCap = 9;  // one bigger to ignore null byte
       for (name, objtype, i) in zip(names, types, 1..) {
         var thisSize: int;
         select objtype.toUpper(): ObjType {