V8 debug helper

This library is for debugging V8 itself, not debugging JavaScript running within V8. It is designed to be called from a debugger extension running within a native debugger such as WinDbg or LLDB. It can be used on live processes or crash dumps, and cannot assume that all memory is available in a dump.

The GDB and LLDB plugins uses a shared a Python FFI bridge over this library and live under tools/debug_helper/plugins/. The WinDbg plugin has different dependencies and lives under tools/windbg/.