[Bug #20755] Frozen string should not be writable via IO::Buffer

	[Bug #20752] Slice of readonly `IO::Buffer` also should be readonly

merge revision(s) c5cf4d4e129f64cb69aaf0a829aed068ef1943c4: [Backport #19542]

	Improve behavioural consistency of unallocated (zero length)
	 `IO::Buffer`. (#9532)

	This makes the behaviour of IO::Buffer.new(0) and IO::Buffer.new.slice(0, 0) consistent.

	Fixes https://bugs.ruby-lang.org/issues/19542 and https://bugs.ruby-lang.org/issues/18805.
	---
	 io_buffer.c                 | 14 ++++++--------
	 test/ruby/test_io_buffer.rb | 35 +++++++++++++++++++++++++++++++++++
	 2 files changed, 41 insertions(+), 8 deletions(-)

	Fix Window private file mapping unlink EACCES issue. (#9358)

	* Don't return early.

	* Add missing `mapping` assignment.

	* Make debug logs conditional.
	---
	 io_buffer.c                 | 18 ++++++++++++------
	 test/ruby/test_io_buffer.rb | 32 ++++++++++++++------------------
	 2 files changed, 26 insertions(+), 24 deletions(-)

* Avoiding using `Tempfile` which was retaining the file preventing it from unlinking.

* Restore experimental warnings.

* Documentation and code structure improvements.

* Improved validation of flags, clarified documentation of argument handling.

* Remove inconsistent use of `Example:` and add example to `null?`.

* Expose `IO::Buffer#private?` and add test.

Benchmark:

```
require "benchmark"

puts(Benchmark.measure do
  ary = 1_000_000.times.map { IO::Buffer.for("") }

  10.times { GC.start(full_mark: false) }
end)
```

Before:

```
14.330119   0.051497  14.381616 ( 14.445106)
```

After:

```
7.481152   0.040166   7.521318 (  7.535209)
```

Since IO::Buffer is allocated using TypedData_Make_Struct, it must use
xfree to free the buffer otherwise it will cause more major GC to run.

Example:

```
10.times do
  1_000_000.times { IO::Buffer.new(0) }

  puts "oldmalloc_increase_bytes: #{GC.stat(:oldmalloc_increase_bytes)}, major_gc_count: #{GC.stat(:major_gc_count)}"
end
```

Before:

```
oldmalloc_increase_bytes: 14904176, major_gc_count: 3
oldmalloc_increase_bytes: 2399424, major_gc_count: 5
oldmalloc_increase_bytes: 5204640, major_gc_count: 6
oldmalloc_increase_bytes: 2199936, major_gc_count: 7
oldmalloc_increase_bytes: 34199936, major_gc_count: 7
oldmalloc_increase_bytes: 24223360, major_gc_count: 8
oldmalloc_increase_bytes: 5967616, major_gc_count: 9
oldmalloc_increase_bytes: 37967616, major_gc_count: 9
oldmalloc_increase_bytes: 9689792, major_gc_count: 10
oldmalloc_increase_bytes: 41689792, major_gc_count: 10
```

After:

```
oldmalloc_increase_bytes: 117392, major_gc_count: 2
oldmalloc_increase_bytes: 26128, major_gc_count: 2
oldmalloc_increase_bytes: 71600, major_gc_count: 2
oldmalloc_increase_bytes: 117072, major_gc_count: 2
oldmalloc_increase_bytes: 17296, major_gc_count: 2
oldmalloc_increase_bytes: 62768, major_gc_count: 2
oldmalloc_increase_bytes: 108240, major_gc_count: 2
oldmalloc_increase_bytes: 153712, major_gc_count: 2
oldmalloc_increase_bytes: 53936, major_gc_count: 2
oldmalloc_increase_bytes: 99408, major_gc_count: 2
```