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Complies with YAML Standard [5.4](https://yaml.org/spec/1.2.2/#54-line-break-characters) [25] instead of matching `\r` only in combination with `\n`.

* fix issue743: handle the empty content of flow sep/map correctly during emitting.

* handle the empty Tag/Anchor properly.

* delete comment

Fixes #501

For completeness I've implemented escaping for characters outside the
basic multilingual plane, but it doesn't get used (as there's no
EscapeAsAsciiJson emitter option implemented).

Inside of a sequence or map, `YAML::Newline` wouldn't reset the collection state, which would cause behavior like this:

```
nodeA:   
    k: [{i: 0},
 {i:1},
  ]NodeB:
    k: [{i: 0},
 {i:1},
  ]
```

* Support kinds of emitter format-setting for YAML::Null.

* update the code and test cases

* add the comment //fallthrough

* fix issue752: generate right long keys

* Improve the readability of test cases

* update to raw string literal

Specifically, printing `.nan`, `.inf`, and `-.inf`, respectively, as per the spec section 10.2.1.4.

The issue is that numbers like
2.01 or 3.01 can not be precisely represented with binary floating point
numbers.

This replaces all occurrences of 'std::numeric_limits<T>::digits10 + 1' with
'std::numeric_limits<T>::max_digits10'.

Background:
Using 'std::numeric_limits<T>::digits10 + 1' is not precise enough.
Converting a 'float' into a 'string' and back to a 'float' will not always
produce the original 'float' value. To guarantee that the 'string'
representation has sufficient precision the value
'std::numeric_limits<T>::max_digits10' has to be used.

Add EmitterStyle, which will allow sequence or map style (i.e., flow or block) to be preserved between parsing and emitting