IO second review: empty File.subPath() allowed, OutputStream.bufferedWriter() added,

normalize(), startsWith(), endsWith() rewritten, LinesStream --> LinesSequence,
File.replaceBytes() / replaceText() rolled back to writeBytes() / writeText(),
some exception messages formulated more clearly, refactorings, comments,
relativeTo() slightly optimized, additional tests

idea/testData/completion/basic/java/NoDuplicationForRuntimeClass.kt

@@ -1,5 +1,5 @@
-val x = LinesStream<caret>
+val x = LinesSequence<caret>
 
 // INVOCATION_COUNT: 2
-// EXIST: { lookupString:"LinesStream", tailText:" (kotlin.io)" }
+// EXIST: { lookupString:"LinesSequence", tailText:" (kotlin.io)" }
 // NUMBER: 1