Implemented ability to deserialize discriminated unions regardless of union tag position

[xperiandri]

I create a snapshot of a reader (a copy of structure) and parse it into a JsonDocument (which is a lazy operation) and do a union case lookup.
I have not added new tests but played with modification of existing (not committed) so add tests as appropriate.

Fixes #93

[xperiandri]

Unfortunately, I was not managed to design changes that would allow doing ValueEquals because readExternalTag requires reading a field name. But all the other cases operate with JsonElement and its value.
Hence I had to get a string instance.

[xperiandri]

@Tarmil could you have a look?

[xperiandri]

@Tarmil

[Tarmil]

I've been pretty busy this week but I'll take a look this weekend.

[Tarmil]

This looks pretty good! I was unsure whether saving and restoring a Utf8JsonReader like this would work, but it seems officially supported, so that's nice.

I added some tests, and fixed an issue in the AdjacentTag case. I'll definitely add a flag to determine whether to use it or not, because despite the low allocation we're still re-parsing the object, and some people may want to ensure they don't do that. But I think I'll make it enabled by default.

[xperiandri]

Cool! Thanks!

[xperiandri]

@Tarmil is there something left preventing this to be merged?

[Tarmil]

Just my schedule 🙂 I'll add the flag and release it now.