Provide a fast-path for decoding leading polymorphic key for streaming

[qwwdfsad]

Basically, we should teach our lexer to (https://github.com/Kotlin/kotlinx.serialization/blob/dev/formats/json/jvmMain/src/kotlinx/serialization/json/internal/JsonLexerJvm.kt#L179) to carefully look until the end of the current buffer in search for polymorphic key