Fix errors in sequence deserialization

[Mingun]

This PR adds a bunch of tests to ensure, that deserialization of sequences is correct in all situations.

This PR is based on #374 and #376, because they make a huge refactoring in files, that this PR touches.

For now all this works:

1. Interleaved sequence fields with non-sequence fields

XML

<any-tag>
  <item/>
  <item/>
  <before/>
  <item/>
  <after/>
  <item/>
</any-tag>

can be deserialized into

struct AnyName {
  before: (),
  item: Vec<()>,
  after: (),
}

2. List of choices

<any-tag>
  <one/>
  <two/>
  <three/>
</any-tag>

can be deserialized into

enum AnyNameChoice {
  one,
  two,
  #[serde(other)]
  other,
}
struct AnyName {
  #[serde(rename = "$value")]
  any_name: Vec<AnyNameChoice>,
}

3. Combination of above

[Mingun]

Replaced by Mingun/fast-xml#12