fixes #74: replaces read_uint_le with utility function

[ndarrell]

resolves #74

Hello! Learning rust, so please excuse me for being misinformed in any way 😄. Any comments or feedback on this would be greatly appreciated!

I had trouble building this off of the default ctap2-2021 branch (did

$ cargo build --example main
# also tried
$ cargo test
``` as the README states, but neither worked), so I tried running the same fix off of `main` where there's similar code. I got a completed run of `cargo test`, but I was not able to get a reproducible failure when just commenting out the old function. I feel I may have missed something with my testing, so, again, any sort of help would be awesome.