Decompilation support

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

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

@@ -0,0 +1,12 @@
+// IntelliJ API Decompiler stub source generated from a class file
+// Implementation of methods is not available
+
+package test
+
+public val p: kotlin.Int /* compiled code */
+
+public fun f(): kotlin.Unit { /* compiled code */ }
+
+private var i: kotlin.Int /* compiled code */
+
+public fun kotlin.Int.plus(i: kotlin.Int /* = compiled code */): kotlin.Int { /* compiled code */ }