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

unmarshaler_test.go

@@ -287,6 +287,54 @@ func TestUnmarshal(t *testing.T) {
 				}
 			},
 		},
+		{
+			desc:  "local datetime into time.Time",
+			input: `a = 1979-05-27T00:32:00`,
+			gen: func() test {
+				type doc struct {
+					A time.Time
+				}
+
+				return test{
+					target: &doc{},
+					expected: &doc{
+						A: time.Date(1979, 5, 27, 0, 32, 0, 0, time.Local),
+					},
+				}
+			},
+		},
+		{
+			desc:  "local datetime into interface",
+			input: `a = 1979-05-27T00:32:00`,
+			gen: func() test {
+				type doc struct {
+					A interface{}
+				}
+
+				return test{
+					target: &doc{},
+					expected: &doc{
+						A: toml.LocalDateTimeOf(time.Date(1979, 5, 27, 0, 32, 0, 0, time.Local)),
+					},
+				}
+			},
+		},
+		{
+			desc:  "local date into interface",
+			input: `a = 1979-05-27`,
+			gen: func() test {
+				type doc struct {
+					A interface{}
+				}
+
+				return test{
+					target: &doc{},
+					expected: &doc{
+						A: toml.LocalDateOf(time.Date(1979, 5, 27, 0, 32, 0, 0, time.Local)),
+					},
+				}
+			},
+		},
 		{
 			desc: "issue 475 - space between dots in key",
 			input: `fruit. color = "yellow"