To avoid crashes during displaying crash reports.

to fix inconsistent and wrong current namespace detections.

This includes:
* Moving load_path and related things from rb_vm_t to rb_namespace_t to simplify
  accessing those values via namespace (instead of accessing either vm or ns)
* Initializing root_namespace earlier and consolidate builtin_namespace into root_namespace
* Adding VM_FRAME_FLAG_NS_REQUIRE for checkpoints to detect a namespace to load/require files
* Removing implicit refinements in the root namespace which was used to determine
  the namespace to be loaded (replaced by VM_FRAME_FLAG_NS_REQUIRE)
* Removing namespaces from rb_proc_t because its namespace can be identified by lexical context
* Starting to use ep[VM_ENV_DATA_INDEX_SPECVAL] to store the current namespace when
  the frame type is MAGIC_TOP or MAGIC_CLASS (block handlers don't exist in this case)

Rather than to lazily check the env using a trinary
value, we can more straightforwardly check for the
env during the VM boot.

This allow `rb_namespace_available` to just be a pointer
dereference.