skip marking for uninitialized imemo_env.

	RUBY_INTERNAL_EVENT_NEWOBJ can expose uninitialized imemo_env
	objects and marking it will cause critical error. This patch
	skips marking on uninitialized imemo_env.

	See: http://rubyci.s3.amazonaws.com/centos7/ruby-master/log/20210329T183003Z.fail.html.gz

	Shortest repro-code is provided by mame-san.
	---
	 gc.c                           | 16 ++++++++++------
	 test/objspace/test_objspace.rb | 13 +++++++++++++
	 2 files changed, 23 insertions(+), 6 deletions(-)

	Implement NameError::message#clone for Ractor

	---
	 bootstraptest/test_ractor.rb   | 13 +++++++++++++
	 error.c                        | 35 +++++++++++++++++++++++++++++++++--
	 test/objspace/test_objspace.rb |  9 +++++++++
	 3 files changed, 55 insertions(+), 2 deletions(-)

RDoc says `ObjectSpace.dump(obj, output: :stdout)   # => nil`,
but it returns STDOUT since fbba6bd4e3dff7a61965208fecae908f10c4edbe.

I think it is unintentional change.

The added test case crashes the interpreter because it makes
ObjectSpace.internal_class_of return the second VALUE slot of an AST
imemo object. The second VALUE slot of `struct rb_ast_struct` is
not a VALUE and not a pointer to a Ruby object.

The two main optimization are:
  - buffer writes for improved performance
  - avoid formatting functions when possible

```

|                   |compare-ruby|built-ruby|
|:------------------|-----------:|---------:|
|dump_all_string    |       1.038|   195.925|
|                   |           -|   188.77x|
|dump_all_file      |      33.453|   139.645|
|                   |           -|     4.17x|
|dump_all_dev_null  |      44.030|   278.552|
|                   |           -|     6.33x|
```

This is useful to see what a block of code allocated, e.g.

```
GC.start
GC.disable
ObjectSpace.trace_object_allocations do
  # run some code
end
gc_gen = GC.count
allocations = ObjectSpace.dump_all(output: :file, since: gc_gen)
GC.enable
GC.start
retentions = ObjectSpace.dump_all(output: :file, since: gc_gen)
```

Compaction makes collisions in the weak map more likely.  For now, clear
the tracing information before the test run so we get accurate results

The allocation tracing code keeps essentially a weak reference to
objects that have been allocated (storing the allocation information
along with the weak ref).  Compacting the heap would break references in
this weak map, so the wrong values could be returned.

This commit just updates the values in the weak ref in order to fix the
allocation tracing book keeping

```
  1) Failure:
TestObjSpace#test_trace_object_allocations [/tmp/ruby/v3/src/trunk-gc_compact/test/objspace/test_objspace.rb:175]:
<nil> expected but was
<"/tmp/ruby/v3/src/trunk-gc_compact/test/objspace/test_objspace.rb">.
```

@tenderlove will re-enable it soon.