This allows to eliminate re-calculation of string lenght and hash value.
See the detailed list of changes in UPGRADING.INTERNALS.

* PHP-8.0:
  Fix #75776: Flushing streams with compression filter is broken

* PHP-7.4:
  Fix #75776: Flushing streams with compression filter is broken

First, the `bzip2.compress` filter has the same issue as `zlib.deflate`
so we port the respective fix[1] to ext/bz2.

Second, there is still an issue, if a stream with an attached
compression filter is flushed before it is closed, without any writes
in between.  In that case, the compression is never finalized.  We fix
this by enforcing a `_php_stream_flush()` with the `closing` flag set
in `_php_stream_free()`, whenever a write filter is attached.  This
call is superfluous for most write filters, but does not hurt, even
when it is unnecessary.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=20e75329f2adb11dd231852c061926d0e4080929>

Closes GH-6703.

We're starting to see a mix between uses of zend_bool and bool.
Replace all usages with the standard bool type everywhere.

Of course, zend_bool is retained as an alias.

* PHP-8.0:
  Fix #80384: limit read buffer size

* PHP-7.4:
  Fix #80384: limit read buffer size

In the case of a stream with no filters, php_stream_fill_read_buffer
only reads stream->chunk_size into the read buffer. If the stream has
filters attached, it could unnecessarily buffer a large amount of data.

With this change, php_stream_fill_read_buffer only proceeds until either
the requested size or stream->chunk_size is available in the read buffer.

Co-authored-by: Christoph M. Becker <cmbecker69@gmx.de>

Closes GH-6444.

This allows reusing an existing zend_string inside a memory stream
without reallocating. For non-readonly streams, the string will
only get separated on write.