Use always File.delete instead of FileUtils.rm

For consistency.
state might not be available at that point
Split display/undisplay to separate files

For consistency.
Remove stepping capabilities in `irb` command

There are several reasons for this:
- `next` command inside `irb` never really worked, because the `next`
keyword in Ruby would get executed instead (see
https://github.com/nixme/pry-nav/issues/20)
- Commands were very limited, they couldn't handle any arguments like
the regular `next`, `step` and `cont` commands.
- It causes issues with the Rails console (see
[https://github.com/deivid-rodriguez/pry-byebug/issues/30).
Standarize debugger_alias test
Remove some unnecessary lines
Remove some TODOs that won't be implemented
Split pry and irb commands into separate files

For consistency.
Fix autoirb test