Piping an alias into other commands fails

[kevsmith]

Given prod-buckets is alias for the pipeline s3:list-buckets --region=us-east-1 corp-prod-* | s3:bucket-info $name:

> prod-buckets | s3:file-info $key

     ** (KeyError) key :redir not found in: %Piper.Command.Ast.PipelineStage{col: 80, 
                                             left: %Piper.Command.Ast.Invocation{...}

[kevsmith]

This happens because Piper.Command.Ast.PipelineStage:new/3 fails to detect the case where two chains of PipelineStages are being concatenated. This results in an incorrectly constructed stage where the left field points to another PipelineStage instead of an Ast.Invocation as it should.