Add function to check if script is cached

The PR is to be able to check to see if OPCache already has a script cached - as it's useful information for autoloaders.

For some class autoloaders (including Composer) it is possible for a script to exist in one of several possible directores. To load that file the autoloader needs to call file_exists e.g. the composer autoloader:

```
foreach ($this->prefixes[$first] as $prefix => $dirs) {
    if (0 === strpos($class, $prefix)) {
        foreach ($dirs as $dir) {
            if (file_exists($dir . DIRECTORY_SEPARATOR . $classPath)) {
                return $dir . DIRECTORY_SEPARATOR . $classPath;
            }
        }
    }
}
```

As PHP doesn't cache the result when the file doesn't exist, this can easily cause several hits to the file system.

Adding the function opcache_script_cached allows the autoloader to check to see if OPCache has already loaded the file in all of the possible locations that where a script could exist, before hitting the filesystem.

```
foreach ($this->prefixes[$first] as $prefix => $dirs) {
    //Check all possible paths in OPCache before checking the file system
    if (0 === strpos($class, $prefix)) {
        foreach ($dirs as $dir) {
            $filename = $dir.DIRECTORY_SEPARATOR.$classPath;
            if (opcache_script_cached($filename) == true) {
                return $filename;
            }
        }

        foreach ($dirs as $dir) {
            $filename = $dir.DIRECTORY_SEPARATOR.$classPath;
            if (file_exists($filename)) {
                return $filename;
            }
        }
    }
}
```

i.e. you can use the information from `opcache_script_cached` to avoid any filesystem hit, once the script is loaded into OPCache.