Suggest using `eq_ignore_ascii_case` on non case-sensitive string comparision

[CrazyboyQCD]

What it does

Lint that suggests using eq_ignore_ascii_case on non case-sensitive string comparision.

Advantage

• avoid extra allocation(example)

Drawbacks

No response

Example

a.to_xx_case() == b.to_xx_case()

Could be written as:

a.eq_ignore_ascii_case(b)

[nyurik]

@rustbot claim

[nyurik]

@CrazyboyQCD hi, I finished the implementation, but it may need some more unit tests - could you add some you could think of? Thx!