Implement bytes-based Unmarshaler interface for tables and arrays (#873)

This change brings back support for the unstable.Unmarshaler interface
for tables and array tables, addressing issue #873.

Key changes:
- Changed UnmarshalTOML signature from (*Node) to ([]byte) to provide
  raw TOML bytes instead of AST nodes
- Added RawMessage type (similar to json.RawMessage) for capturing raw
  TOML bytes for later processing
- Updated handleKeyValuesUnmarshaler to reconstruct key-value lines
  from the parsed keys and raw value bytes
- Added support for slice types implementing Unmarshaler (e.g., RawMessage)
- Removed unused AST helper functions from unstable/ast.go

The bytes-based interface allows users to:
- Get raw TOML bytes for custom parsing
- Delay TOML decoding using RawMessage
- Implement custom unmarshaling logic for complex types

Tests added for:
- Table unmarshaler with various scenarios
- Array table unmarshaler
- Split tables (same parent defined in multiple places)
- RawMessage usage
- Nested tables and mixed regular fields

unstable/ast.go

@@ -143,49 +143,3 @@ func (n *Node) Value() *Node {
 func (n *Node) Children() Iterator {
 	return Iterator{nodes: n.nodes, idx: n.child}
 }
-
-// SetNodeSlice sets the backing nodes slice for a node.
-// This is used when building synthetic nodes.
-func SetNodeSlice(n *Node, nodes *[]Node) {
-	n.nodes = nodes
-}
-
-// SetNodeChild sets the child index for a node.
-// This is used when building synthetic nodes.
-func SetNodeChild(n *Node, child int32) {
-	n.child = child
-}
-
-// SetNodeNext sets the next sibling index for a node.
-// This is used when building synthetic nodes.
-func SetNodeNext(n *Node, next int32) {
-	n.next = next
-}
-
-// GetNodeChild returns the child index for a node.
-// This is used when copying nodes.
-func GetNodeChild(n *Node) int32 {
-	return n.child
-}
-
-// GetNodeNext returns the next sibling index for a node.
-// This is used when copying nodes.
-func GetNodeNext(n *Node) int32 {
-	return n.next
-}
-
-// GetNodeIndex returns the index of node n in the backing slice,
-// using relativeTo's nodes slice as reference.
-// Returns -1 if n is not in the slice.
-func GetNodeIndex(n *Node, relativeTo *Node) int32 {
-	if relativeTo.nodes == nil || n == nil {
-		return -1
-	}
-	nodes := *relativeTo.nodes
-	for i := range nodes {
-		if &nodes[i] == n {
-			return int32(i)
-		}
-	}
-	return -1
-}