[PATCH] Rename to `struct rbimpl_size_overflow_tag`

	This struct is used for addition not only for multiplication, so
	remove the word `mul`, and make the member names more descriptive.

	[PATCH] IO::Buffer: Fill the test for `IO::Buffer#clear`

	[PATCH] [Bug #21787] IO::Buffer: Check addition overflows

	https://hackerone.com/reports/3437743

The allocated buffers may be consecutive memory addresses. This will mean
that `b->base == a->base + a->size` even though `a` and `b` are separate
buffers.

Use PR_SET_VMA_ANON_NAME to set human-readable names for anonymous
virtual memory areas mapped by `mmap()` when compiled and run on Linux
5.17 or higher.  This makes it convenient for developers to debug mmap.

[Feature #20902]

The current implementation of `IO::Buffer#copy` and `#set_string` has
an undefined behavior when the source and destination memory overlaps,
due to the underlying use of the `memcpy` C function.

This patch guarantees the methods to be safe even when copying between
overlapping buffers by replacing `memcpy` with `memmove`,

Fixes: [Bug #20745]

This does not change any actual behaviour, but provides a choke point for blocking IO operations.

* Update `IO::Buffer` to use `rb_io_blocking_region`.

* Update `File` to use `rb_io_blocking_region`.

* Update `IO` to use `rb_io_blocking_region`.

Update documentation.