Decompilation support

- class header reader for KotlinMultifileClass, KotlinMultifileClassPart
- proper implClassName for multifile class members

idea/testData/decompiler/decompiledTextJvm/MultifileClass/MultifileClass.kt

@@ -0,0 +1,10 @@
+@file:JvmName("MultifileClass")
+@file:JvmMultifileClass
+package test
+
+private var i = 2
+
+fun Int.plus(i: Int = 1) = this + i
+
+class ShouldNotBeVisible1
+interface ShouldNotBeVisible2