Show that Tapioca is not stuck

[Morriar]

For some (very large) gems Tapioca can take a while to generate the RBI file.

While Tapioca is processing thousands of constants and methods, from the user point of view it may seem like nothing is happening and Tapioca is stuck:

Compiled previous_gem
      create  sorbet/rbi/gems/[email protected]
# Nothing for a few minutes...

We should change the output to indicate that something is indeed happening a reassure the user that Tapioca isn't stuck.

Maybe we could display which constant/property we're currently compiling:

Compiling RBI for [email protected]
  constant1 # this line would be refreshed with the current thing being compiled to avoid too much verbosity
            # then when the file is finished we can replace it by `created  sorbet/rbi/gems/[email protected]`