Newline in keys should be escaped

[abadger]

Hi I was looking at the tomlkit behaviour with newlines in keys and found a bug:

>>> import tomlkit
>>> tomlkit.__version__
'0.5.11'
>>> print(tomlkit.dumps({'a\nb': 'a\nb'}))
"a
b" = "a\nb"

The toml spec says that the key, equal sign, and value must be on the same line (the value can span multiple lines if it is a multiline string but it must start on the same line as the other parts). ( https://toml.io/en/v1.0.0#keyvalue-pair )

I asked for clarification of what should happen with newlines in key values on the toml lang github and the consensus was that the escaped form of a newline should be used instead of the actual newline: toml-lang/toml#826

Thus the desired behaviour would be for tomlkit.dumps do this:

>>> print(tomlkit.dumps({'a\nb': 'a\nb'}))
"a\nb" = "a\nb"

[srinivasreddy]

Another relevant discussion. toml-lang/toml#835