non-deterministic builds

[ilmanzo]

Hello, while working on reproducible builds for openSUSE,
I found that this crate does produce different build results in every clean build because generated content varies randomly.

I created a reproducer here: https://github.com/ilmanzo/rust_i18n_repro_test/tree/main

Probably the main reason of non-determinism is the use of an HashMap, where iteration order is random, so translation assets ends in the binary in a different order on each build.

I'd suggest to use a BTreeMap to preserve iteration order among the assets, if you agree I can work on a PR as well :)

Thanks