add [as_underscore] lint

[DevAccentor]

closes #8847

detect usage of as _ and enforce the usage of explicit type like

fn foo(n: usize) {}
let n: u16 = 256;
foo(n as _);

will suggest to change to

fn foo(n: usize) {}
let n: u16 = 256;
foo(n as usize);

changelog: add [as_underscore] lint

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Manishearth (or someone else) soon.

Please see the contribution instructions for more information.

[Manishearth]

@bors r+

pretty straightforward, thanks!

[bors]

📌 Commit 64fe4e3 has been approved by Manishearth

[bors]

⌛ Testing commit 64fe4e3 with merge ebd357e...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing ebd357e to master...