Added bundled proguard rules

[shanshin]

Resolves #1121
Resolves #1899
Resolves #1900
Resolves #2050

[sandwwraith]

Thank you for the thorough investigation!

[bubenheimer]

I think an additional README comment should be added regarding Proguard/R8 usage with obfuscation. In this case serialized class and field names will be obfuscated, and will be non-stable. Personally I use @SerialName annotations to preserve serialized name stability, but others might opt for additional keep rules. (Just don't make those default rules, please!)

[sandwwraith]

Serialized class names are stored in the descriptor, so it shouldn't affect polymorphic serialization, only error messages would be altered

[bubenheimer]

The new README reads as if the new default Proguard rules took care of any minimization/obfuscation issues, but that's not true. Obfuscated names are not stable, so the next app build will not be able to read back some of the serialized data, at least with JSON serialization. Pretty sure I tested this.

By default, proguard rules are supplied with the library. These rules keep serializers for all serializable classes that are retained after shrinking, so you don't need additional setup.

Ideally the README could mention the problem with obfuscation and highlight various potential solutions, like @SerialName and custom user keep rules and annotations.