feat(extension) Implement wasm_get_memory_buffer

[Hywan]

Sequel of #35.
Depends on wasmerio/wasmer#296.

Implement wasm_get_memory_buffer:

function wasm_get_memory_buffer(resource $wasm_instance): ?WasmArrayBuffer;

Usage example:

$bytes = wasm_fetch_bytes('my_program.wasm');
$instance = wasm_new_instance($bytes);
$pointer = wasm_invoke_function($instance, 'function_returning_a_pointer_to_a_string', []);

// Get a memory buffer.
$memory = wasm_get_memory_buffer($instance);

// Get a view over the memory buffer.
$view = new WasmUint8Array($memory, $pointer);

// Read the memory to, for instance, read a NUL-terminated ASCII string.
$nth = 0;

while (0 !== $view[$nth]) {
    echo chr($view[$nth]);
    ++$nth;
}

echo "\n";