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)

Distinguish between platform-dependent ".rbbin" and platform-
independent ".inc" files.

The order of iseq may differ from the order of tokens, typically
`while`/`until` conditions are put after the body.

These orders can match by using line numbers as builtin-indexes, but
at the same time, it introduces the restriction that multiple `cexpr!`
and `cstmt!` cannot appear in the same line.

Another possible idea is to use `RubyVM::AbstractSyntaxTree` and
`node_id` instead of ripper, with making BASERUBY 3.1 or later.

when the RUBY_FREE_ON_SHUTDOWN environment variable is set, manually free memory at shutdown.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>
Co-authored-by: Peter Zhu <peter@peterzhu.ca>