ext4 implementation

[arihant2math]

The most probable order of completion, since each one requires the previous in order to be useful.

• Initial implementation (Read-only) Initial ext4 support #64
• Metadata modification Some support for ext4 writing #145
• Modifying files without changing their size Some support for ext4 writing #145
• Expanding or contracting file size (without block allocation/deallocation) ext4: Implement truncation and expansion without block allocation #170
• Directory modification (Partially with Bump dependencies + more ext4 writing #232, no htree support)
• htree directory modification
• File creation Bump dependencies + more ext4 writing #232
• File deletion Bump dependencies + more ext4 writing #232
• Expanding or contracting file size with block allocation/deallocation - block maps
• Expanding or contracting file size with block allocation/deallocation - extents Bump dependencies + more ext4 writing #232
• xattr support
• journaling support
• publish to crates.io

[arihant2math]

Got inode allocation working! Unfortunately adding inodes to directories isn't working yet.

Once we have directory modification we'll be able to:

• symlink (<60 characters only)
• hardlink
• create empty files

as for block allocation/freeing: I have a WIP implementation of growable files for block maps, but most ext4 filesystems use extents, which are much harder to support.