Do not store instantiated classes for clients anywhere

It turned out this is not needed. Clents will just be eval'd every time
they will be called via WFM.
Put statements from client toplevel into |main|, not |initialize|

This change will allow us to translate |return| statements at the client
toplevel as plain Ruby |return| statements that will return from the
|main| method. With |initialize| this wasn't possible, because its
return value is ignored.
Compile |return| statements at client toplevel