Minor, unindent code
Simplify code in StackValue$CollectionElement
Simplify function invocation in ExpressionCodegen

Callee is now generated only when it could be non-trivial, which is only the
case when generating function call in invokeFunction()
Minor, FieldInfo.toString()
Include error type in error message

Everything besides it is almost always useless: descriptor is usually
ERROR_CLASS, declarations are usually nulls because they come from a different
module
Minor refactoring in callable reference codegen

Don't store an extra field for functionDescriptor, since it's available from
the superclass as "callableDescriptor". Store a referenced function descriptor
instead
Support local function references in codegen

 #KT-3704 In Progress
 #KT-1183 In Progress
Add tests on callable references

Object members and accessors for private class members
Support closures in codegen for callable references
Support references to local extensions in codegen