allocate<T extends NativeType> 抽象方法

在本地堆上分配 `byteCount` 字节的内存。

如果提供了 `alignment`，则分配的内存至少将对齐到 `alignment` 字节数。

为了分配 `sizeOf<T>()` 字节的多倍，请直接将分配器作为函数调用：`allocator<T>(count)`（有关详细信息，请参阅 AllocatorAlloc.call）。

// This allocates two bytes. If you intended two Int32's, this is an
// error.
allocator.allocate<Int32>(2);

// This allocates eight bytes, which is enough space for two Int32's.
// However, this is not the idiomatic way.
allocator.allocate<Int32>(sizeOf<Int32>() * 2);

// The idiomatic way to allocate space for two Int32 is to call the
// allocator directly as a function.
allocator<Int32>(2);

如果字节数或对齐无法满足，将抛出 `ArgumentError`。