Expose a way to iterate over `DeEvent`s

[iliazeus]

Right now, there are two main ways to use this library:

• a rather low-level one, using a Reader
• a very high-level one, with serde

I would like to have an in-between option, one that still has a pull-based interface, but that would still do a lot of the low-level stuff by itself - like gluing together and trimming Texts, for example.

From what I understand, such an interface already exists internally - I mean the de::XmlReader::next() function, which satisfies all of the above. However, it is not currently exposed publicly, even though the DeEvents themselves are.

[Mingun]

Yes, I plan to do such intermediate interface, I outlined a very brief sketch of it in https://github.com/tafia/quick-xml/pull/766#issuecomment-2198637734