Small example using boxing and partial application results in throwing `Error`.

[fridis]

This is somewhat realted to #3308 and #3314. This code

f(x ()->unit) =>
s(x Any) =>
say (f (s unit))

produces

 > ./build/bin/fz partial_bug.fz 
Missing handling of class dev.flang.ast.This class dev.flang.ast.ExprWithPos in LibraryOut.expressions
Missing handling of class dev.flang.ast.This class dev.flang.ast.ExprWithPos in LibraryOut.expressions

error 1: java.lang.Error: class dev.flang.ast.This should no longer exist at runtime
    at dev.flang.air.Clazzes.clazz(Clazzes.java:1004)
    at dev.flang.air.Clazzes.findClazzes(Clazzes.java:742)
    at dev.flang.air.Clazz$EV.action(Clazz.java:169)
    at dev.flang.ast.Expr.visitExpressions(Expr.java:338)
    at dev.flang.ast.AbstractCall.visitExpressions(AbstractCall.java:99)
    at dev.flang.ast.Box.visitExpressions(Box.java:155)
    at dev.flang.ast.AbstractCall.visitExpressions(AbstractCall.java:93)
    at dev.flang.ast.AbstractAssign.visitExpressions(AbstractAssign.java:139)
    at dev.flang.ast.AbstractFeature.visitExpressions(AbstractFeature.java:1594)
    at dev.flang.air.Clazz.inspectCode(Clazz.java:1534)
    at dev.flang.air.Clazz.findAllClasses(Clazz.java:1578)
    at dev.flang.air.Clazzes.findAllClasses(Clazzes.java:408)
    at dev.flang.fuir.FUIR.<init>(FUIR.java:187)
    at dev.flang.opt.Optimizer.fuir(Optimizer.java:73)
    at dev.flang.tools.Fuzion$Backend.processFrontEnd(Fuzion.java:506)
    at dev.flang.tools.Fuzion.lambda$parseArgsForBackend$3(Fuzion.java:1071)
    at dev.flang.tools.Tool.lambda$run$0(Tool.java:141)
    at dev.flang.util.Errors.runAndExit(Errors.java:957)
    at dev.flang.tools.Tool.run(Tool.java:141)
    at dev.flang.tools.Fuzion.main(Fuzion.java:626)

*** fatal errors encountered, stopping.
one error.

[fridis]

A slightly extended example might help to give some more insight:

f(x ()->unit) => _ := x.call
a is
  redef as_string => "--a--"
  s(x Any) => say x
  say (f (s a.this))

_ := a

results in

 > ~/fuzion/work/build/bin/fz reg_issue3308a.fz

error 1: Found error clazz in set of clazzes in the IR even though no earlier errors were reported.  This can only be the result of a severe bug.

one error.

The problem seems to be that on the call to s a.this, an attempt is made to box a.fun.call and not a.fun.call.outer.outer, resulting in chaos.