Support empty if-statements

Type-check "if (...) ;" to Unit, report "implicit cast to Unit", propagate data
flow info out of its condition

 #KT-2478 Fixed
Support codegen of empty while-loop

 #KT-3009 In Progress
Support codegen of empty do-while-loop

 #KT-3009 In Progress
Support codegen of empty for-loop

 #KT-3009 Fixed
 EA-35742 fixed
Don't show bytecode for files out of source roots (EA-50793)
Fix ClassCastException with nullable chars

 #KT-4210 Fixed
 #KT-4098 Fixed
 #KT-4251 Fixed
Don't fail when using Unit class object as a value

 #KT-4243 Fixed
Don't assert that class is found in BindingContext

This isn't true when light classes are generated, according to other such
checks in the same file

 #KT-4276 Fixed
Resolve generic bounds of all classes prior to supertypes
Print the entire bytecode upon ASM verification failure
Unextend Char from Number

This is impossible to achieve on JVM, where java/lang/Character does not extend
java/lang/Number
Move declaration of jet.Char out of Numbers.kt

It's not a number anymore (see previous commit)