Auto merge of rust-lang#12283 - nyurik:ref-format-args, r=xFrednet

Minor refactor format-args

* Move all linting logic into a single format implementations struct

This should help with the future format-args improvements.

**NOTE TO REVIEWERS**:  use "hide whitespace" in the github diff -- most of the code has shifted, but relatively low number of lines actually modified.

Followig up from rust-lang#12274

r? `@xFrednet`

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changelog: none

Author: bors