Rework BOM handling and encoding API

[Mingun]

• Fixes encoding feature breaks API #180
• Fixes quick-xml captures UTF BOM as Event::Text #191. @TakaakiFuruse, could you look at this, whether it solves your problems properly?
• Fixes reduce code redundancy for the encoding feature #262

This PR solves several highly-coupled problems, that blocks changing for safe namespace handling (#59). The main reason in that various decode methods accepts a reference to the Reader, but if we move namespace buffer inside the Reader, we could face with borrowing problems (actually, namespace buffer was decoupled from Reader due to this reason in #69).

First, I've fixed #191 by introducing a new StartText event which generated when reader encounters "text" before the XML markup. In well-formed XMLs that text could represent a BOM.

Secondly, I've removed all functions that handle BOM in inappropriate time. For example, there a no sense to handle BOM when you decode attribute values, because you will never get BOM there. For that reason I've moved all BOM handling code to the BytesStartText struct.

Because after that change you can handle BOM only as the first event from the reader, that allowed me to move the automatic BOM-based encoding detection to the reader instead of triggering it by calling BytesText::decode_without_bom (where the name is also incorrect, since the function removes BOM from text and decodes the rest content, so correct one is decode_with_bom_removal).

As encoding API already changed very much, I've also solved the #180 by unify function signature with and without encoding feature enabled.

[codecov-commenter]

Codecov Report

Merging #399 (d49a4b8) into master (3b37c0e) will decrease coverage by 0.42%.
The diff coverage is 75.20%.

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
- Coverage   61.72%   61.29%   -0.43%     
==========================================
  Files          20       20              
  Lines       10233    10148      -85     
==========================================
- Hits         6316     6220      -96     
- Misses       3917     3928      +11     

Flag	Coverage Δ
unittests	61.29% <75.20%> (-0.43%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/de/escape.rs	65.15% <ø> (-1.28%)	⬇️
src/errors.rs	9.52% <0.00%> (-2.85%)	⬇️
src/events/attributes.rs	94.19% <0.00%> (+3.52%)	⬆️
src/lib.rs	21.09% <0.00%> (-4.92%)	⬇️
src/reader.rs	88.38% <79.44%> (-0.02%)	⬇️
src/de/mod.rs	76.14% <80.00%> (+0.87%)	⬆️
src/events/mod.rs	75.57% <80.85%> (+1.23%)	⬆️
src/de/seq.rs	91.83% <100.00%> (-0.76%)	⬇️
src/se/mod.rs	93.81% <100.00%> (-0.01%)	⬇️
src/writer.rs	90.36% <100.00%> (+0.02%)	⬆️
... and 6 more

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[dralley]

You do a great job with testing, documentation, splitting up commits and such - I can't find too much to complain about :)

All the changes here look sensible

[Mingun]

@dralley, I tried to reword some sentences in changelog as best as I can, and also add a couple of comments to the TagState. Could you take a look at whether is all ok?

[dralley]

I left some suggestions

[Mingun]

Thanks! In the final version I've updated benchmarks (they has been broken since #393) and applied you suggestions. Merging now