Embed structure members directly, rather than allocated separately.

This reduces the number of allocations and indirection.

Instead of 

``` c
typedef struct _php_ds_map_t {
    zend_object  std;
    ds_map_t    *map;
} php_ds_map_t;
```

use 

``` c
typedef struct _php_ds_map_t {
    zend_object  std;
    ds_map_t     map;
} php_ds_map_t;
```
