Some nullsafety improvements, type fixes and a lot of new Kotlin 1.4 extensions

• #141 Fix: requireNoNulls() now converts T? to T

• 1df6e1a Fix: .dart and .iter on Iterable<T> now work for all types not only if T implements Comparable

• f43cbc5 Fix: *NotNull methods now return non-nullable types

• b727893 Fix: The hashcode of all collections doesn't get cached anymore. That caused problems when mutable items in a KtList changed. The equals and hashCode methods now always change together.

• #141 Improve: KtIterable.onEach can now be chained

• #141 New: KtIterable.cast() to manually cast values

• #141 New: KtIterable.onEachIndexed

• #163 New: KtIterable.shuffled()

• #166 New: KtIterable.reduceOrNull()

• #169 New: KtMutableList.removeFirst() KtMutableList.removeLast()

• #173 New: KtMutableList.removeFirstOrNull() KtMutableList.removeLastOrNull()

• #171 New: KtIterable.minOf()

• #165 New: String.replaceFirstChar()

• #170 New: KtIterable.minOrNull(), KtIterable.maxOrNull(), deprecates KtIterable.min(), KtIterable.max(),

• #174 New: KtIterable.runningReduce()

• #177 New: KtCollection.randomOrNull()

• #178 New: KtIterable.flatMapIndexed, KtIterable.flatMapIndexedTo

Thanks to the #hacktoberfest contributors @Anas35, @robiness, @MatthaiosSait, @Dev-dfm, @marciokuroki, @Rishabh-Negi