Re-design Utf8JsonWriter as a regular class rather than a ref struct. (dotnet/corefx...

Re-design Utf8JsonWriter as a regular class rather than a ref struct. (dotnet/corefx#36961)

* Add an in-box array-backed IBufferWriter<T>

* Update Utf8JsonWriter ref and main writer file.

* Fix JsonWriter WriteValue APIs.

* Use Environment.NewLine static and invert some stream conditions.

* Update JsonWriter properties and fix serializer build breaks.

* Update JsonWriter unit tests.

* Add xml comments, clean up, and improve test coverage.

* Update JsonDocument and JsonSerializer to react to JsonWriter changes.

* Normalize the reference assembly.

* Do not escape/validate comments and update issue number.

* Do not escape comments and validate they don't contain embedded
delimiter

* Remove dead code and update issue number in comments.

* Throw InvalidOEx instead of ArgEx if IBW doesn't give requested memory.

* Fix test build breaks for netfx.

* Remove dead code and fix source packaging.

* Address PR feedback.

* Disable writing floats test on windows

* 8 digit floats don't work well on older TFMs. Reduce to 7.

Commit migrated from https://github.com/dotnet/corefx/commit/1f9b84a0804e868c7e0f37a3c10fbaf7c735ae14