Pry::CodeObject: remove unnecessary 'target' parameter

Since we can access 'target' via _pry_.current_context

* We also update a few files (code_collector/show_info) that need to be updated for this change
edit command: add --method option

* --method doesn't accept any parameters, it only edits the 'current' method associated
with the binding. It's equivalent to 'edit-method' (with no args).
* This option is necessary as 'edit' by itself edits the last expression.
* Note that --method doesn't just edit the current method, if no method exists (i.e binding.pry was not put
inside a method context) then it edits the current OBJECT instead.

TODO:
* explicitly restrict --method to *just* methods and not allow it to fall back to objects?
* allow --method to take a parameter that must be a method object? e.g --method Pry#repl works
but --method Pry would fail ?