Fix poor formatting of impls

[topecongiro]

This PR fixes two minor formatting bugs w.r. impls:

1. Currently rustfmt does not respect empty_item_single_line config option when formatting empty impls.
2. Putting the where on the same line with the indented closing bracket, which makes the following where clauses hard to read (Poor formatting of impl header with long type and where bound #2746).

Closes #2746.

[nrc]

Thanks!