Auto merge of rust-lang#13592 - MihailMihov:trait_impl_assist, r=Veykril

Add assist to generate trait impl's

resolves rust-lang#13553

This pull request adds a `generate_trait_impl` assist, which generates trait impl's for a type. It is almost the same as the one to generate impl's and I also reduced the trigger range to only outside the `RecordFieldList`. Also moved all the tests into separate test functions. A few of the old tests seemed redundant, so I didn't port them.

Author: bors