Make sort stable (#416)

Sort reverses records that compare as equal, making it unstable.  Fix
that by using the proper comparison in expr.(*RecordSlice).Less.

This reverts #37.

expr/sort.go

@@ -103,7 +103,7 @@ func (r *RecordSlice) Swap(i, j int) { r.records[i], r.records[j] = r.records[j]
 
 // Less implements sort.Interface for *Record slices.
 func (r *RecordSlice) Less(i, j int) bool {
-	return r.sorter(r.records[i], r.records[j]) <= 0
+	return r.sorter(r.records[i], r.records[j]) < 0
 }
 
 // Push adds x as element Len(). Implements heap.Interface.

tests/suite/sort/sort-4-1.yaml

@@ -14,6 +14,6 @@ output: |
   0:[a;]
   0:[b;]
   0:[c;]
+  0:[-;]
   #1:record[notS:string]
   1:[bleah;]
-  0:[-;]

tests/suite/sort/sort-4-2.yaml

@@ -10,10 +10,10 @@ input: |
   1:[bleah;]
 
 output: |
-  #0:record[notS:string]
-  0:[bleah;]
-  #1:record[s:string]
-  1:[-;]
-  1:[a;]
-  1:[b;]
-  1:[c;]
+  #0:record[s:string]
+  0:[-;]
+  #1:record[notS:string]
+  1:[bleah;]
+  0:[a;]
+  0:[b;]
+  0:[c;]

tests/suite/sort/sort-4-3.yaml

@@ -14,6 +14,6 @@ output: |
   0:[c;]
   0:[b;]
   0:[a;]
+  0:[-;]
   #1:record[notS:string]
   1:[bleah;]
-  0:[-;]

tests/suite/sort/sort-4-4.yaml

@@ -10,10 +10,10 @@ input: |
   1:[bleah;]
 
 output: |
-  #0:record[notS:string]
-  0:[bleah;]
-  #1:record[s:string]
-  1:[-;]
-  1:[c;]
-  1:[b;]
-  1:[a;]
+  #0:record[s:string]
+  0:[-;]
+  #1:record[notS:string]
+  1:[bleah;]
+  0:[c;]
+  0:[b;]
+  0:[a;]