WIP: Prepare next major release of Thoth.Json ecosystem

[MangelMaxime]

Changes:

• Rewrite the core library in order to build the path on the Error path it should increase performance when there is no problem #43
• Mutualise code between the different implementation
• Allow to override even the default type when using Auto modules It will allow people to have full control over the JSON representation of their type: Fix https://github.com/thoth-org/Thoth.Json.Net/issues/31
• Mirror Fable behaviour when Decoding "StringEnum" UnionCase: Fix https://github.com/thoth-org/Thoth.Json.Net/issues/30
• Support single union case on Newtonsoft and Fable side: https://github.com/thoth-org/Thoth.Json/issues/42 The JSON representation has been changed for these ones so if people were storing their JSON it can't be decoded anymore by default. We made the decoder support both the new style and old style "for free" using oneOf so it should be backwards compatible when decoding a JSON. The encoding will only use the new style as it would be strange of people to really on it to generates a specific JSON. They should use manual encoders for these cases.
• Add char support: https://github.com/thoth-org/Thoth.Json/issues/36
• Add support for 'T seq #26
• Add support for mutable dictionary
• Add support for mutable hashset