So that it's possible to create an Environment not yet attached
to any V8 context. We'll use this to deserialize the V8 context
before attaching an Environment to it.

Add an ExternalReferenceRegistry class for registering static
external references.

To register the external JS to C++ references created in a binding
(e.g. when a FunctionTemplate is created):

- Add the binding name (same as the id used for `internalBinding()`
  and `NODE_MODULE_CONTEXT_AWARE_INTERNAL`) to
  `EXTERNAL_REFERENCE_BINDING_LIST` in `src/node_external_reference.h`.
- In the file where the binding is implemented, create a registration
  function to register the static C++ references (e.g. the C++
  functions in `v8::FunctionCallback` associated with the function
  templates), like this:

  ```c++
  void RegisterExternalReferences(
          ExternalReferenceRegistry* registry) {
    registry->Register(cpp_func_1);
  }
  ```
- At the end of the file where `NODE_MODULE_CONTEXT_AWARE_INTERNAL` is
  also usually called, register the registration function with

  ```
  NODE_MODULE_EXTERNAL_REFERENCE(binding_name,
                                 RegisterExternalReferences);
  ```

Pass the flags down to node_mksnapshot so that we can use them
when generating the snapshot (e.g. to debug or enable V8 flags)

This includes the initial Environment (without running bootstrap
scripts) into the builtin snapshot

Keep track of snapshotted modules in JS land, and move
bootstrap switches into StartExecution() so that
they are not included into part of the environment-independent
bootstrap process.

This runs `lib/internal/bootstrap/loaders.js` before creating
the builtin snapshot and deserialize the loaders from the
snapshot in deserialization mode.

The connection between the JS land zero fill toggle and the
C++ one in the NodeArrayBufferAllocator gets lost if the toggle
is deserialized from the snapshot, because V8 owns the underlying
memory of this toggle. This resets the connection at pre-execution.

Fast APIs need to work with ArrayBuffers which we need
to rebuild connections to after deserializing them
from the snapshot. For now, postpone their creation
until pre-execution to simplify the process.

Since V8 snapshot does not currently support instance member
initialization, initialize them in ordianry class constructors
for now so that these classes can be included in the snapshot.
This may be reverted once
https://bugs.chromium.org/p/v8/issues/detail?id=10704
is fixed and backported.

This runs `lib/internal/bootstrap/node.js` before creating
the builtin snapshot and deserialize the loaders from the
snapshot in deserialization mode.