Refactor writer process to support direct disk writing

[ifd3f]

This closes #10, #115, and #124:

• Re-enable O_DIRECT on Linux #10: we add O_DIRECT back in with this PR.
• Decompressing progress shown as burning, and no progress on actual burning #115: we write a status message after the final flush to fix the timeseries offset.
• Failed to write block to disk: Os { code: 22, kind: InvalidInput, message: "Invalid argument" }) on MacOS #124: we fix this by writing whole blocks, because MacOS complains if you don't.

We also significantly improve the unit testing of the writer process.

Test plan

Test plan, multiple screenshots

Aligned file, Linux

> cargo run -- burn test.img -z none -s none -f --root always
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.10s
     Running `target/debug/caligula burn test.img -z none -s none -f --root always`
Input file: test.img
Detected compression format: no compression
> Select target disk sdc | STORAGE DEVICE - 62.6 GB (disk, remov
able: yes)

Unaligned file, Linux

> cargo run -- burn LICENSE -z none -s none -f --root always
   Compiling caligula v0.4.4 (/home/astrid/Documents/caligula)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.65s
     Running `target/debug/caligula burn LICENSE -z none -s none -f --root always`
Input file: LICENSE
Detected compression format: no compression
> Select target disk sdc | STORAGE DEVICE - 62.6 GB (disk, removable: yes)
[sudo] password for astrid: 

Aligned compressed file, Linux

> cargo run -- burn duo256_sd.img.lz4 -s none -f --root always
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.09s
     Running `target/debug/caligula burn duo256_sd.img.lz4 -s none -f --root always`
Input file: duo256_sd.img.lz4
Detected compression format: lz4
> Select target disk sdc | STORAGE DEVICE - 62.6 GB (disk, remov
able: yes)

Aligned file, MacOS

> cargo run -- burn test.img -z none -s none -f --root always
warning: caligula@0.4.4: native/darwin/REDiskList.m:30:13: warning: 'NSArray' may not respond to 'sortUsingSelector:'
warning: caligula@0.4.4:     [_disks sortUsingSelector:@selector(localizedCaseInsensitiveCompare:)];
warning: caligula@0.4.4:      ~~~~~~ ^
warning: caligula@0.4.4: 1 warning generated.
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.16s
     Running `target/debug/caligula burn test.img -z none -s none -f --root always`
Input file: test.img
Detected compression format: no compression
> Select target disk disk2 | Generic STORAGE DEVICE Media - 62.6 GB (disk, remov
able: yes)

Unaligned file, MacOS

cargo run -- burn LICENSE -z none -s none -f --root always
warning: caligula@0.4.4: native/darwin/REDiskList.m:30:13: warning: 'NSArray' may not respond to 'sortUsingSelector:'
warning: caligula@0.4.4: [_disks sortUsingSelector:@selector(localizedCaseInsensitiveCompare:)];
warning: caligula@0.4.4: ~~~~~~ ^
warning: caligula@0.4.4: 1 warning generated.
Finished dev profile [unoptimized + debuginfo] target(s) in 0.17s
Running target/debug/caligula burn LICENSE -z none -s none -f --root always
Input file: LICENSE
Detected compression format: no compression
Select target disk disk2 | Generic STORAGE DEVICE Media - 62.6 GB (disk, remov
able: yes)

Performance impact?

As it turns out, negligible!

The following tests are run on the main branch, with uncompressed 1GB files.

Naively running Caligula on the base branch on Linux seems to indicate that this PR causes performance degradation.

However, that's because we aren't properly syncing the file, since std::io::File::flush is a no-op. Actually adding in a .sync_data() at the end of the write indicates that the disk is technically not fully written by that point, so the actual write speed is slower than it looks from the program's end.

These speeds are more in line with what we saw above.

[ifd3f]

giving it a look-over and thorough testing in the morning when i'm not tired. we wouldn't want to merge bad code here :)