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 API change proposal for a next version #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 Numbers are encoded as strings Thoth.Json.Net#31
• Mirror Fable behaviour when Decoding "StringEnum" UnionCase: Fix Thoth.Json.Net cannot find Union case Thoth.Json.Net#30
• Support single union case on Newtonsoft and Fable side: Q: Transparently support erased single-case DUs on .NET side #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: Missing support for System.Char on auto decoder #36
• Add support for 'T seq No encoder/decoder for 'T seq #26
• Add support for mutable dictionary
• Add support for mutable hashset