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>

unstable/parser.go

@@ -69,8 +69,8 @@ func (p *Parser) Data() []byte {
 // panics.
 func (p *Parser) Range(b []byte) Range {
 	return Range{
-		Offset: uint32(p.subsliceOffset(b)), //nolint:gosec // TOML documents are small
-		Length: uint32(len(b)),              //nolint:gosec // TOML documents are small
+		Offset: uint32(SubsliceOffset(p.data, b)), //nolint:gosec // TOML documents are small
+		Length: uint32(len(b)),                    //nolint:gosec // TOML documents are small
 	}
 }
 
@@ -82,20 +82,6 @@ func (p *Parser) rangeOfToken(token, rest []byte) Range {
 	return Range{Offset: uint32(offset), Length: uint32(len(token))} //nolint:gosec // TOML documents are small
 }
 
-// subsliceOffset finds the byte offset of subslice b within p.data
-// by scanning for the matching element address.
-func (p *Parser) subsliceOffset(b []byte) int {
-	if len(b) == 0 {
-		return len(p.data)
-	}
-	for i := range p.data {
-		if &p.data[i] == &b[0] {
-			return i
-		}
-	}
-	panic("subslice is not within parser data")
-}
-
 // Raw returns the slice corresponding to the bytes in the given range.
 func (p *Parser) Raw(raw Range) []byte {
 	return p.data[raw.Offset : raw.Offset+raw.Length]