Builtins: made Unit object.
JVM backend: fixed the getting Unit instance -- Unit.VALUE -> Unit.instance$.
Minor: fixed tests.
Minor in IDEA plugin: jet -> kotlin in JetBundle.properties
IDEA plugin: fixed highlighting for objects.
Minor corrections in JVM codegen

- use mapSignature instead of mapToCallableMethod where possible
- use resolvedCall where possible
- fix some warnings and style issues
Minor cleanup in ExpressionCodegen#pushMethodArguments
Don't use ExpressionCodegen's internals in MonitorInstruction
Introduce an abstract ArgumentGenerator in ExpressionCodegen
Inline ExpressionCodegen#pushMethodArguments, drop "skipLast" hack
Avoid duplicate synthetic parameter marking in FunctionCodegen
Refine type of CalculatedClosure#superCall

Instead of PSI, store the actual resolved call
Minor, rename "inner" -> "nested" in tests
Minor, add assertion message to CodegenContext
Minor, beautify code in StackValue.CallReceiver
Minor, delete outdated comments and refine type
Don't fail on a bad Retention annotation in light classes

 #KT-5450 Fixed

No new tests added because it seems there are no tests on highlighting which
cause light class generation, where the original error occurred
Java to Kotlin converter: fixed conversion of access to Kotlin property declared as constructor parameter
Java to Kotlin converter: minor code corrections after review
Java to Kotlin converter: checking that no element prototype assignment forgotten + no memory leaks through singleton Element instances