Command-line support for running scalac/dotty to completion (variance checks, .class)

[vkuncak]

Right now sbt plugin allows stainless to run stainless files through the subsequent phases and generate code. However, it appears that this functionality is not available from command line.

There are two reasons why I wish that it to be available (when requested by an option):

1. we want to make sure that stainless verified code is accepted by Scala. This is in particular so given that certain type checks such as variance are ran after transformations that we do not follow (presumably because they are after type checker).

2. we may want to run the compiled code because running code is fun and it is convenient to be able to use the same script for verification and running.

By default, I believe there is no point in running the phases when we have any sort of verification or termination checking error. The common case is that there is an error, so running checks and code generation should not slow down the common case, only the last (successful) run.

[vkuncak]

Ghost elimination is also something that we would thus get from command line.