Pry::Command::Cat::FileFormatter: make format() private
Reimplemented play, save-file with Pry::Command::CodeCollector

Pry::Command::CodeCollector is a mixin that extracts out shared
functionality from play, save-file and gist. Gist is yet to be
reimplemented.
Pry::CodeObject: binding_lookup --> empty_lookup
implement whereami -n (exclude line numbers)
Pry::Command::CodeCollector: replace module inclusion with delegation

And implement play and save-file using the new delegator. 'gist' still needs
to be re-implemented using CodeCollector.

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TODO:
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Couldn't write a test for `play --lines 4..5` with implied target.eval("__FILE__") parameter,
need to come up with a decent test.