Dev/reproducible ordering

[apognu]

This relates to #171.

This simple pull request enables the consistent ordering of the files embedded in the binary, to allow for consitent and reproducible builds. This uses a built-in method in walkdir. I wrote a local test to check behavior, but this relies or disorderfs which randomize entry ordering in a FUSE filesystem, so did not push it (it also only is relevant is run many times, and I don't know how to structure that with the macro used for Asset).

For reference, the test looked like this:

use rust_embed::RustEmbed;

#[derive(RustEmbed)]
#[folder = "/tmp/target"]
struct Asset;

#[test]
fn consistent_ordering() {
  let order: Vec<String> = ["a", "aa", "b", "ba", "bb", "c", "ec"].iter().map(|f| f.to_string()).collect();
  let provided: Vec<String> = Asset::iter().map(|f| f.to_string()).collect();

  assert_eq!(provided, order, "ordering should be equal");
}

I had to fix the example for poem that didn't seem to pass with the latest version the Cargo.toml resolves to, mainly:

• The whole server and listeners are now behind a feature flag.
• The signature for Endpoint::call() seems to have changed.

I don't know if you would prefer another PR for this, or if I can keep it mutualized into this one.

[AzureMarker]

It looks like the authors of poem don't understand semver. I opened poem-web/poem#246, so hopefully going forward they keep downstream users in mind. Thanks for updating the example.

[pyrossh]

LGTM