dumpfile: Add some test code to convert to tar

[cgwalters]

Add code to go from dumpfile -> tar (notably, ignoring non-inline regular file content). This is testing only code right now, though clearly mapping to/from tar would be very useful in general. I plan to use this code for various testing purposes especially to manually create tarballs from various dumpfiles to e.g. feed directly to mkfs.erofs for example.

But we at least get some sanity checking this way.

[jeckersb]

👍 Looks good to me, pretty straightforward, just a few nits from latest clippy (1.81), most important being the MSRV one:

warning: this import is redundant
  --> rust/composefs/examples/cfs-dumpfile2tar.rs:14:1
   |
14 | use composefs;
   | ^^^^^^^^^^^^^^ help: remove it entirely
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_component_path_imports
   = note: `#[warn(clippy::single_component_path_imports)]` on by default

warning: this expression creates a reference which is immediately dereferenced by the compiler
  --> rust/composefs/examples/cfs-dumpfile2tar.rs:66:36
   |
66 |                 pax_data.write_all(&value)?;
   |                                    ^^^^^^ help: change this to: `value`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
   = note: `#[warn(clippy::needless_borrow)]` on by default

warning: useless conversion to the same type: `u64`
  --> rust/composefs/examples/cfs-dumpfile2tar.rs:95:29
   |
95 |             let rdev: u64 = (*rdev).into();
   |                             ^^^^^^^^^^^^^^ help: consider removing `.into()`: `(*rdev)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_conversion
   = note: `#[warn(clippy::useless_conversion)]` on by default

warning: current MSRV (Minimum Supported Rust Version) is `1.70.0` but this item is stable since `1.77.0`
   --> rust/composefs/examples/cfs-dumpfile2tar.rs:150:15
    |
150 |     let dst = File::create_new(dst)
    |               ^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
    = note: `#[warn(clippy::incompatible_msrv)]` on by default

warning: `composefs` (example "cfs-dumpfile2tar") generated 4 warnings (run `cargo clippy --fix --example "cfs-dumpfile2tar"` to apply 3 suggestions)

[cgwalters]

Good catch, I had to upgrade from 1.80 -> 1.81 to get that new clippy check.
Also the way it rewrote with "a".repeat() is way nicer.