Consolidate subslice offset into a single SubsliceOffset function

Remove the private subsliceOffset methods from both parser.go and
errors.go. Replace them with a single exported SubsliceOffset function
in ast.go (next to the Range type it serves).

SubsliceOffset finds the byte offset by comparing element addresses:
&data[i] == &subslice[0]. This is well-defined Go pointer comparison
on elements of the same backing array.

This fixes the v2.3.0 regression (#1047) where the parser's
subsliceOffset used len(data) - len(b), which only works for suffix
slices, not arbitrary subslices like error highlights. It also removes
the reflect-based implementation from errors.go.

Fixes #1047

Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>

errors.go

@@ -99,7 +99,7 @@ func (e *DecodeError) Key() Key {
 //
 //nolint:funlen
 func wrapDecodeError(document []byte, de *unstable.ParserError) *DecodeError {
-	offset := subsliceOffset(document, de.Highlight)
+	offset := unstable.SubsliceOffset(document, de.Highlight)
 
 	errMessage := de.Error()
 	errLine, errColumn := positionAtEnd(document[:offset])
@@ -261,17 +261,3 @@ func positionAtEnd(b []byte) (row int, column int) {
 
 	return row, column
 }
-
-// subsliceOffset finds the byte offset of subslice within data by
-// scanning for the matching element address.
-func subsliceOffset(data []byte, subslice []byte) int {
-	if len(subslice) == 0 {
-		return len(data)
-	}
-	for i := range data {
-		if &data[i] == &subslice[0] {
-			return i
-		}
-	}
-	panic("subslice is not within data")
-}