AST Tweaks (#551)

* Use pointers instead of copying around ast.Node

Node is a 56B struct that is constantly in the hot path. Passing nodes
around by copy had a cost that started to add up. This change replaces
them by pointers. Using unsafe pointer arithmetic and converting
sibling/child indexes to relative offsets, it removes the need to carry
around a pointer to the root of the tree. This saves 8B per Node. This
space will be used to store an extra []byte slice to provide contextual
error handling on all nodes, including the ones whose data is different
than the raw input (for example: strings with escaped characters), while
staying under the size of a cache line.

* Remove conditional

* Add Raw to track range in data for parsed values

* Simplify reference tracking

parser_test.go

@@ -9,7 +9,6 @@ import (
 
 //nolint:funlen
 func TestParser_AST_Numbers(t *testing.T) {
-
 	examples := []struct {
 		desc  string
 		input string
@@ -136,7 +135,6 @@ func TestParser_AST_Numbers(t *testing.T) {
 	for _, e := range examples {
 		e := e
 		t.Run(e.desc, func(t *testing.T) {
-
 			p := parser{}
 			p.Reset([]byte(`A = ` + e.input))
 			p.NextExpression()
@@ -167,7 +165,7 @@ type (
 	}
 )
 
-func compareNode(t *testing.T, e astNode, n ast.Node) {
+func compareNode(t *testing.T, e astNode, n *ast.Node) {
 	t.Helper()
 	require.Equal(t, e.Kind, n.Kind)
 	require.Equal(t, e.Data, n.Data)
@@ -199,7 +197,6 @@ func compareIterator(t *testing.T, expected []astNode, actual ast.Iterator) {
 
 //nolint:funlen
 func TestParser_AST(t *testing.T) {
-
 	examples := []struct {
 		desc  string
 		input string
@@ -338,7 +335,6 @@ func TestParser_AST(t *testing.T) {
 	for _, e := range examples {
 		e := e
 		t.Run(e.desc, func(t *testing.T) {
-
 			p := parser{}
 			p.Reset([]byte(e.input))
 			p.NextExpression()