Add 'inner' keyword

Support for the keyword in PSI and stubs

 #KT-1174 In Progress
Add ClassDescriptor.isInner()

Implement in resolve & lazy resolve; place a stub in resolve-java

 #KT-1174 In Progress
Inaccessible outer class member is now an error

 #KT-1174 In Progress
Labeled this & super to nested class' outer is an error

 #KT-1174 In Progress
Forbid usage of outer's generic parameter in nested class

 #KT-1174 In Progress
Check all modifiers for objects

DeclarationsChecker didn't check visibility modifiers for objects
Report ILLEGAL_MODIFIER on inappropriate 'inner' keyword usage

 #KT-1174 In Progress
Disallow nested classes within inner & local classes

 #KT-1174 In Progress
DescriptorRendererImpl now writes 'inner' for inner classes

 #KT-1174 In Progress
Remove unused method
Remove 'inner enum' hack & fix Java inner class resolve

Inner enum was placed into the class object of the outer class. Remove the
hack from frontend, frontend.java & backend. Fix tests

 #KT-1174 In Progress
Get rid of MutableClassDescriptorLite.isStatic()

isStatic() is now precisely !isInner()
Support inner/nested classes in JVM back-end

 #KT-1174 In Progress
Fix completion for nested class qualified expressions

 #KT-1174 In Progress
(TODO) Resolve qualified expressions in type checker
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