feat(zipWithNext, pairwise): add zipWithNext, add `pairwise(transfo…

…rm)`

Related issue: Kotlin/kotlinx.coroutines#1767

src/commonMain/kotlin/com/hoc081098/flowext/utils/NULL_VALUE.kt

@@ -36,5 +36,12 @@ import kotlin.jvm.JvmField
 @JvmField
 public val NULL_VALUE: Symbol = Symbol("NULL_VALUE")
 
+/**
+ * This is a work-around for having nested nulls in generic code.
+ * This allows for writing faster generic code instead of using `Option`.
+ * This is only used as an optimisation technique in low-level code.
+ *
+ * This is internal and should not be used outside of the library.
+ */
 @JvmField
 internal val INTERNAL_NULL_VALUE: Symbol = Symbol("INTERNAL_NULL_VALUE")