fmap support

Adds basic fmap support to redoxfs. Work in progress PR to make sure I'm not doing things completely incorrectly so I don't waste more days on this approach if so.

Basic idea:

• Read file to buffer
• Write file back to buffer on fsync

Limitations:

• There is a max amount of fmaps that can be open. This is because the memory can't reallocate - old references would be invalidated.
• Fmaps are shared, but only if their offsets/lengths fit within a previous fmap's (rounded to the page size). This is once again because nothing can be reallocated and therefore invalidated.
• Because they aren't always shared, it's unspecified which copy they will share with.
• Fmaps are not synced on funmap. This would be doable once the kernel sends a scheme event for that.

Current Limitations (that can be fixed sort of):

• Files are only written to with an explicit fsync. This could be fixed with some sort of timer or something idk

This example now works