Generate not-null assertions after method calls

If a method comes from Java and is annotated as returning NotNull, after
calling it we should check if it actually returned something other than null.
Introduce checkReturnedValueIsNotNull() in jet/runtime/Intrinsics which does
exactly that.

CallableMethod's invoke() and invokeDefault() are now private, use asserted
versions instead

compiler/testData/codegen/notNullAssertions/A.java

@@ -0,0 +1,28 @@
+import jet.runtime.typeinfo.KotlinSignature;
+
+public class A {
+    @KotlinSignature("fun foo() : String")
+    public String foo() {
+        return null;
+    }
+    
+    @KotlinSignature("fun staticFoo() : String")
+    public static String staticFoo() {
+        return null;
+    }
+    
+    @KotlinSignature("fun plus(a: A) : A")
+    public A plus(A a) {
+        return null;
+    }
+
+    @KotlinSignature("fun inc() : A")
+    public A inc() {
+        return null;
+    }
+    
+    @KotlinSignature("fun get(o: Any) : Any")
+    public Object get(Object o) {
+        return null;
+    }
+}