Change xml crate

[MarcoIeni]

I am working on this issue and I noticed that this library is really slow if compared to what I built by using quick-xml.

I am assuming that the difference of performances is due to the different xml library used.

If you want to compare the two solutions:

git clone -b riir https://github.com/MarcoIeni/svdtools
cd svdtools

# try version the version with quick-xml 
git checkout be257c679028d10c6d86e0ae9c869a0ee421cb7c
cargo build --release
time ./target/release/svdtools mmap example/stm32l4x2.svd

# try the version that uses svd-parser
git checkout fad7abfd923312ec95f307286a05017be1218217
cargo build --release
time ./target/release/svdtools mmap example/stm32l4x2.svd

These are the results I get on my laptop by running the executable about ten times:

quick-xml version:
minimum: 0.03s user 0.02s system 79% cpu 0.066 total
maximum: 0.08s user 0.01s system 87% cpu 0.100 total

svd-parser version:
minimum: 0.26s user 0.02s system 96% cpu 0.291 total
maximum: 0.30s user 0.02s system 95% cpu 0.332 total

Of course the version I built by using quick-xml is incomplete and doesn't handle errors well (I originally wrote it because I didn't know this crate existed), but I just wanted to tell you that I saw this huge performance difference.

Of course I know that these benchmark are not rigorous, but I think that they are sufficient at least to raise this point to your attention.

[burrbull]

svd-parser now uses roxmltree which even faster
svd-encoder still uses xmltree