Provide tab-completion for aliases

[lexmag]

Closes #2519.

It is based on IEx.Server.whereis/0 usage, as there is no other way to reach current %IEx.State{}.
Probably we can use separate agent to track state, not sure if it worth it though; the current implementation might be OK for a first iteration.
I'd like to have any feedback or suggestions ot it. Just for reference: #2519 (comment).

This also makes possible to add tab-completion for user imports.

[josevalim]

isn't it clearer to make it aliases ++ Enum.map(...)?

[josevalim]

Thank you! I have added some comments!

[lexmag]

Thanks for your comments.
It turned out that proposed implementation not includes one case:

iex(1)> alias IEx, as: I
nil
iex(2)> IEx.
App                Autocomplete       CLI                Config             
Evaluator          Helpers            History            Introspection      
Remsh              Server             State              after_spawn/0      
after_spawn/1      color/2            configuration/0    configure/1        
inspect_opts/0     pry/1              pry/3              started?/0         
width/0            
iex(2)> I.  
after_spawn/0      after_spawn/1      color/2            configuration/0    
configure/1        inspect_opts/0     pry/1              pry/3              
started?/0 

note the lack of modules for I. return. Will rework PR to support it.

[lexmag]

@josevalim this is ready for merge. Don't see a way to make tests for it (we can start IEx with predefined %Env{} but capture_io swaps group_leader thus IEx.Server is getting unreachable).
Here is a showcase:

iex(1)> alias IEx, as: I
nil
iex(2)> I
I          IEx        IO         Inspect    Integer    
iex(2)> I.
App                Autocomplete       CLI                Config             
Evaluator          Helpers            History            Introspection      
Remsh              Server             State              after_spawn/0      
after_spawn/1      color/2            configuration/0    configure/1        
inspect_opts/0     pry/1              pry/3              started?/0         
width/0            
iex(2)> I.App.start
start/2

[josevalim]

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