Do not allow consequent primitives in $value fields and top-level

[Mingun]

This PR changes serialization of $value fields with types like Vec<String> or (u32, u32, u32). Previously this fields would be serialized without delimiters between elements which is unlikely what you want. Now serialization of that types would return SeError::Unsupported.

Example:

struct Xml {
  #[serde(rename = "$value")]
  seq: Vec<String>,

  #[serde(rename = "$value")]
  tuple: (bool, usize, String),
}

Currently it is based on #820, only the last commit implements the change.

This also partially handle this #497 (comment).

[dralley]

Might be helpful to mention the "consequent primitives" caveat in the documentation.

[Mingun]

I think, it is already mentionen in $value/Primitives and sequences of primitives part of documentation (but I forgot to update this part, now fixed). If you have better place where we can put it or describe it better, make a PR. Perhaps my gaze is too blurry.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 82.92683% with 7 lines in your changes missing coverage. Please review.

Project coverage is 60.21%. Comparing base (39b5905) to head (a9391f3).
Report is 22 commits behind head on master.

Files with missing lines	Patch %	Lines
src/de/mod.rs	0.00%	7 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #823      +/-   ##
==========================================
+ Coverage   60.08%   60.21%   +0.12%     
==========================================
  Files          41       41              
  Lines       15975    16021      +46     
==========================================
+ Hits         9599     9647      +48     
+ Misses       6376     6374       -2     

Flag	Coverage Δ
unittests	60.21% <82.92%> (+0.12%)	⬆️

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