Improve XmlVersion enumeration: explicit and implicit 1.0 variants

[Mingun]

This PR changes XmlVersion from

pub enum XmlVersion {
  V1_0,
  V1_1,
}

to

pub enum XmlVersion {
  Implicit1_0,
  Explicit1_0,
  Explicit1_1,
}

The motivation behind this change is simplification with processing versions. XML specification explicitly allows at most one XML declaration (the <?xml version=...?> part). At the same time the version information in the declaration is mandatory. This leads us to a simple cycle that allows, using one variable, to track the uniqueness of the declaration and obtain values according to the version

let mut reader = Reader::from_str(...);
let mut version = XmlVersion::Implicit1_0;
loop {
  match reader.read_event() {
    // Return error in case of duplicated declaration
    Event::XmlDecl(e) if version == XmlVersion::Implicit1_0 => return Err(...),
    // Return error in case of missing "version"
    Event::XmlDecl(e) => { version = e.xml_version()?; }

    Event::Text(e) => {
      let text = e.xml_content(version)?;
      // ...
    }
    ...
  }
}

Without thus changes you may use Option<XmlVersion>, but then you will forced to use e.xml_content(version.unwrap_or(XmlVersion::V1_0)), which will add check for None for every usage of version in the loop. Usually XML documents a well-formed, so this is just wasting of CPU cycles.

XmlVersion is not a part of any release yet, so it can be changed freely.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 72.07207% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.47%. Comparing base (a759d65) to head (c0f371d).
⚠️ Report is 18 commits behind head on master.

Files with missing lines	Patch %	Lines
src/events/attributes.rs	84.48%	9 Missing ⚠️
examples/custom_entities.rs	0.00%	6 Missing ⚠️
examples/read_nodes.rs	0.00%	5 Missing ⚠️
benches/macrobenches.rs	0.00%	4 Missing ⚠️
src/events/mod.rs	62.50%	3 Missing ⚠️
src/lib.rs	85.00%	3 Missing ⚠️
src/de/mod.rs	83.33%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #955      +/-   ##
==========================================
+ Coverage   55.08%   56.47%   +1.38%     
==========================================
  Files          44       44              
  Lines       16911    17648     +737     
==========================================
+ Hits         9316     9966     +650     
- Misses       7595     7682      +87     

Flag	Coverage Δ
unittests	56.47% <72.07%> (+1.38%)	⬆️

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