Raw FIR: implement expression trees & conversions from PSI

Testing: total kotlin test controls no stubs in FIR in non-stub-mode
#KT-29002 Fixed

idea/testData/fir/multiModule/basic/m2_java_dep(m1-java)/user.txt

@@ -1,4 +1,4 @@
 FILE: user.kt
     public final function foo(hello: R|hello/Hello|): R|kotlin/String| {
-        STUB
+        return@@@foo STUB
     }