Extend needless_lifetimes to suggest eliding impl lifetimes

[smoelius]

Example:

error: the following explicit lifetimes could be elided: 'a
  --> tests/ui/needless_lifetimes.rs:332:10
   |
LL |     impl<'a> Foo for Baz<'a> {}
   |          ^^              ^^
   |
help: elide the lifetimes
   |
LL -     impl<'a> Foo for Baz<'a> {}
LL +     impl Foo for Baz<'_> {}

The main change is in how impl lifetime uses are tracked. Previously, a hashmap was created, and lifetimes were removed from the hashmap as their uses were discovered. However, the uses are needed to generate elision suggestions. So, now, uses are added to the hashmap as they are discovered.

The PR is currently organized as six commits, which I think are self-explanatory:

• Extend needless_lifetimes to suggest eliding impl lifetimes
• Reorder functions [not strictly necessary, but IMHO, the code is better structured as a result]
• Fix lifetime tests
• Fix non-lifetime tests
• Fix clippy_lints and clippy_utils
• Fix typo in needless_lifetimes test

r? @Alexendoo (I think you are needless_lifetimes' primary author? Sorry if I have this wrong.)

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changelog: Extend needless_lifetimes to suggest eliding impl lifetimes