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

internal/danger/danger.go

@@ -0,0 +1,65 @@
+package danger
+
+import (
+	"fmt"
+	"reflect"
+	"unsafe"
+)
+
+const maxInt = uintptr(int(^uint(0) >> 1))
+
+func SubsliceOffset(data []byte, subslice []byte) int {
+	datap := (*reflect.SliceHeader)(unsafe.Pointer(&data))
+	hlp := (*reflect.SliceHeader)(unsafe.Pointer(&subslice))
+
+	if hlp.Data < datap.Data {
+		panic(fmt.Errorf("subslice address (%d) is before data address (%d)", hlp.Data, datap.Data))
+	}
+	offset := hlp.Data - datap.Data
+
+	if offset > maxInt {
+		panic(fmt.Errorf("slice offset larger than int (%d)", offset))
+	}
+
+	intoffset := int(offset)
+
+	if intoffset > datap.Len {
+		panic(fmt.Errorf("slice offset (%d) is farther than data length (%d)", intoffset, datap.Len))
+	}
+
+	if intoffset+hlp.Len > datap.Len {
+		panic(fmt.Errorf("slice ends (%d+%d) is farther than data length (%d)", intoffset, hlp.Len, datap.Len))
+	}
+
+	return intoffset
+}
+
+func BytesRange(start []byte, end []byte) []byte {
+	if start == nil || end == nil {
+		panic("cannot call BytesRange with nil")
+	}
+	startp := (*reflect.SliceHeader)(unsafe.Pointer(&start))
+	endp := (*reflect.SliceHeader)(unsafe.Pointer(&end))
+
+	if startp.Data > endp.Data {
+		panic(fmt.Errorf("start pointer address (%d) is after end pointer address (%d)", startp.Data, endp.Data))
+	}
+
+	l := startp.Len
+	endLen := int(endp.Data-startp.Data) + endp.Len
+	if endLen > l {
+		l = endLen
+	}
+
+	if l > startp.Cap {
+		panic(fmt.Errorf("range length is larger than capacity"))
+	}
+
+	return start[:l]
+}
+
+func Stride(ptr unsafe.Pointer, size uintptr, offset int) unsafe.Pointer {
+	// TODO: replace with unsafe.Add when Go 1.17 is released
+	//   https://github.com/golang/go/issues/40481
+	return unsafe.Pointer(uintptr(ptr) + uintptr(int(size)*offset))
+}