remove `unproven` flag

[burrbull]

Encoding works fine as for me.

I think we should prove it and remove unproven option.

Alternative:
Split this crate on several like svd-base, svd-parser, svd-encoder, svd-utils, tests.

[marcoieni]

I think we should prove it and remove unproven option.

This means writing some tests, right? :)

In my project I added a test in which I test this library.
In the test I decode an xml, I encode it and then I decode it again.
The test asserts that the two decoded xml are equal.
Here is the test:
https://github.com/MarcoIeni/svdtools/blob/riir/tests/svd_parser.rs

Maybe we could start by writing a (integration?) test like this.