Provide support for JsonNamingStrategy

[sandwwraith]

to be used in Json for enums and properties (configured separately)

Provide basic implementations: AllLowercase and SnakeCase

Fixes #33

[sandwwraith]

After some consideration and discussions, we've decided that enum parsing is an entirely different feature from naming strategy: while strategy changes names, enum parsing should affect values. Besides, there are some cases when the strategy for enums is not enough: e.g. you want to parse both Foo and foo into FOO, like Jackson's CASE_INSENSETIVE_ENUMS.

Therefore, enum support would be removed entirely from this PR. I'll reopen #209 for it.

[sandwwraith]

@qwwdfsad @shanshin It's ready for review

[pdvrieze]

The serialName parameter could possibly be renamed to elementSerialName as there is also a type serial name. (The documentation here, especially the guarantee in code, make the semantics clear though. The parameter still does present a redundant parameter (as it can be retrieved trivially).

[sandwwraith]

In the initial prototype, there was only descriptor and serialName; you can easily also say that elementIndex is redundant, as you can get it via descriptor.getElementName(). However, I think it is easier to provide both of them to avoid hassle

[sandwwraith]

Or, it is possible, to name it elementName instead 🤔

[Angel-Ponce]

How can i use this feature?
You probably need to drop an example when pushed new features, in my opinion the main documentation page it's a little bit poor

[sschuberth]

How can i use this feature?

Like:

import kotlinx.serialization.json.JsonNamingStrategy

val json = Json {
    ignoreUnknownKeys = true
    namingStrategy = JsonNamingStrategy.SnakeCase
    serializersModule = module
}

Edit: ignoreUnknownKeys = true and serializersModule = module are not relevant, they were just part of my code.

[Angel-Ponce]

How can i use this feature?

Like:

import kotlinx.serialization.json.JsonNamingStrategy

val json = Json {
    ignoreUnknownKeys = true
    namingStrategy = JsonNamingStrategy.SnakeCase
    serializersModule = module
}

Thank you so much!

[sandwwraith]

All the reference (including this example) is located in our guide: https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/json.md#global-naming-strategy