This implements two optimizations on fs::Path, which N added in 12.0.0.

The current structure looks like: 

```cpp
struct Path {
    const char *m_str; // Points to the read-only path string
    char *m_write_buffer_buffer; // Part of std::unique_ptr<char[], ams::fs::impl::Deleter>
    ams::fs::impl::Deleter m_write_buffer_deleter; // Parse of std::unique_ptr<char[], ams::fs::impl::Deleter>, stores the size of the buffer.
    size_t m_write_buffer_length; // Copy of the write buffer's size accessible to the Path() structure.
    bool m_is_normalized; // Whether the path buffer is normalized
};
```

This is pretty wasteful. The write buffer size is stored twice, wasting 8 bytes, because one copy of the size isn't accessible to the path.

In addition, due to alignment, the bool wastes 7 padding bytes.

This commit:

* Encodes normalized in the low bit of the write buffer length, saving 8 bytes.
* Use a custom WriteBuffer class rather than generic unique_ptr, to avoid needing to store the WriteBuffer twice.


These each save 8 bytes, for a final size of 0x18 rather than 0x28.