Ability to pick PHP value by reference in FFI.

Description

HI!
Currently the only way convert php value to FFI value is memcpy (or other ways with memory copying involved).
It could be wasting if we for example received large chunk from socket and need to deal with it through FFI.
For example,

/* Initial */
echo memory_get_usage(); // 348504

/* Got chunk of data */
$data = str_repeat('1', 512 << 10);
echo memory_get_usage(); // 876992

/* Defining the FFI type */
$type = FFI::arrayType(FFI::type('char'), [512 << 10]);
echo memory_get_usage(); // 877184

/* Allocating memory for type */
$ffiValue = FFI::new($type);
echo memory_get_usage(); // 1401984

/* And memcpy */
FFI::memcpy($ffiValue, $data, 512 << 10);
echo memory_get_usage(); 1402048

It would be great to have ability to reflect value from PHP to FFI using it by reference like so:

$data = str_repeat('1', 512 << 10);

/* @var FFI\CData $ffiValue */
$ffiValueCharArray = FFI::represent('char[]', &$data, size: 512 << 10); // No additional memory allocations for $data content

$ffiValueCharArray [1] = 5;
var_dump($data[1]); // $data changed by reference

$ffiValueInt32 = FFI::represent('int32_t', &$data, size: 4, offset: 12);
var_dump($ffiValueInt32->cdata); // substr($data, 12, 4) === "1111", we will get here 825307441

$ffiValueInt32->cdata = 10;
var_dump(bin2hex(substr($data, 12, 4))); // "0a000000"

Also could work not only with strings:

$int = 10;

$ffiValueInt64 = FFI::represent('int64_t', &$int); // length argument could be omitted if cast performed to fixed size type
var_dump($ffiValueInt64->cdata); // 10;

$ffiValueInt64->cdata = 20;
var_dump($int); // 20;

$readOverBoundary = FFI::represent('char[]', &$int, 10); // Exception: attempt to read over boundary

从示例中展示的 FFI::type、FFI::arrayType、FFI::new 和 FFI::memcpy 入口点开始，然后确定所提议的引用语义是否符合 PHP 的值和内存模型。达到由维护者批准的 FFI::represent 拟议行为设计，即表示完成。