Add CachedIterable (rewindable, allows any key&repeating keys)

1. This eagerly evaluates any iterable,
   and stores a read only copy of the keys and values.
2. This allows using any value as a key, and allows repeating keys.

For example, this can be used to store the exact result of keys and values
yielded by a generator.

Longer-term, this may be useful in creating internal helper functions/methods such as
`someprefix_map(iterable $iterable): CachedIterable`, `someprefix_flip(iterable $iterable): CachedIterable`,

```php
function my_generator() {
    yield '123' => 'numeric string is preserved';
    yield '123' => 'allows repeating keys';
    yield new stdClass() => 'false';
}
$iterable = new CachedIterable(my_generator());
foreach ($iterable as $key1 => $value1) {
    foreach ($iterable as $key2 => $value2) { ... }
}
```

```php
final class CachedIterable implements IteratorAggregate, Countable
{
    public function __construct(iterable $iterator) {}
    public function getIterator(): InternalIterator {}
    public function count(): int {}
    public static function fromPairs(array $pairs): CachedIterable;

    public function __serialize(): array {}  // [$k1, $v1, $k2, $v2,...]
    public function __unserialize(array $data): void {}

    // useful for converting iterables back to values
    public function keys(): array {}
    public function values(): array {}
    // useful to efficiently get offsets at the middle/end of a long iterable
    public function keyAt(int $offset): mixed {}
    public function valueAt(int $offset): mixed {}
    // dynamic properties are forbidden
}
```

Author: TysonAndre

ext/spl/config.m4

@@ -1,4 +1,4 @@
-PHP_NEW_EXTENSION(spl, php_spl.c spl_functions.c spl_engine.c spl_iterators.c spl_array.c spl_directory.c spl_exceptions.c spl_observer.c spl_dllist.c spl_heap.c spl_fixedarray.c, no,, -DZEND_ENABLE_STATIC_TSRMLS_CACHE=1)
-PHP_INSTALL_HEADERS([ext/spl], [php_spl.h spl_array.h spl_directory.h spl_engine.h spl_exceptions.h spl_functions.h spl_iterators.h spl_observer.h spl_dllist.h spl_heap.h spl_fixedarray.h])
+PHP_NEW_EXTENSION(spl, php_spl.c spl_functions.c spl_engine.c spl_iterators.c spl_array.c spl_directory.c spl_exceptions.c spl_observer.c spl_dllist.c spl_heap.c spl_fixedarray.c spl_cachediterable.c, no,, -DZEND_ENABLE_STATIC_TSRMLS_CACHE=1)
+PHP_INSTALL_HEADERS([ext/spl], [php_spl.h spl_array.h spl_directory.h spl_engine.h spl_exceptions.h spl_functions.h spl_iterators.h spl_observer.h spl_dllist.h spl_heap.h spl_fixedarray.h spl_cachediterable.h])
 PHP_ADD_EXTENSION_DEP(spl, pcre, true)
 PHP_ADD_EXTENSION_DEP(spl, standard, true)

ext/spl/config.w32

@@ -1,5 +1,5 @@
 // vim:ft=javascript
 
-EXTENSION("spl", "php_spl.c spl_functions.c spl_engine.c spl_iterators.c spl_array.c spl_directory.c spl_exceptions.c spl_observer.c spl_dllist.c spl_heap.c spl_fixedarray.c", false /*never shared */, "/DZEND_ENABLE_STATIC_TSRMLS_CACHE=1");
+EXTENSION("spl", "php_spl.c spl_functions.c spl_engine.c spl_iterators.c spl_array.c spl_directory.c spl_exceptions.c spl_observer.c spl_dllist.c spl_heap.c spl_fixedarray.c spl_cachediterable.c", false /*never shared */, "/DZEND_ENABLE_STATIC_TSRMLS_CACHE=1");
 PHP_SPL="yes";
-PHP_INSTALL_HEADERS("ext/spl", "php_spl.h spl_array.h spl_directory.h spl_engine.h spl_exceptions.h spl_functions.h spl_iterators.h spl_observer.h spl_dllist.h spl_heap.h spl_fixedarray.h");
+PHP_INSTALL_HEADERS("ext/spl", "php_spl.h spl_array.h spl_directory.h spl_engine.h spl_exceptions.h spl_functions.h spl_iterators.h spl_observer.h spl_dllist.h spl_heap.h spl_fixedarray.h spl_cachediterable.h");

ext/spl/php_spl.c

@@ -33,6 +33,7 @@
 #include "spl_observer.h"
 #include "spl_dllist.h"
 #include "spl_fixedarray.h"
+#include "spl_cachediterable.h"
 #include "spl_heap.h"
 #include "zend_exceptions.h"
 #include "zend_interfaces.h"
@@ -722,6 +723,7 @@ PHP_MINIT_FUNCTION(spl)
 	PHP_MINIT(spl_dllist)(INIT_FUNC_ARGS_PASSTHRU);
 	PHP_MINIT(spl_heap)(INIT_FUNC_ARGS_PASSTHRU);
 	PHP_MINIT(spl_fixedarray)(INIT_FUNC_ARGS_PASSTHRU);
+	PHP_MINIT(spl_cachediterable)(INIT_FUNC_ARGS_PASSTHRU);
 	PHP_MINIT(spl_observer)(INIT_FUNC_ARGS_PASSTHRU);
 
 	return SUCCESS;