Calling rb_current_namespace() in rb_namespace_current() means to show
the definition namespace of Namespace.current itself (it's the root always)
but the users' expectation is to show the namespace of the place where
the Namespace.current is called.

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)

This reverts the following commits as it's causing OOM in some cases in
ruby/ruby.wasm test suite.
* 372515f33c908b36b3f5fbd2edcb34c69b418500
* 3a730be8b464454878a42132f6fecb98ab4c1b5b

Previously, Ruby displayed backtraces for each thread on deadlock. However, it has not been shown since Ruby 3.0.
It should display the backtrace for debugging.

Co-authored-by: Jeremy Evans <code@jeremyevans.net>

It has been used only for DOSISH other than Windows.