Only single out std. Put everything else in a consistent list.

…ednet

new lint `legacy_numeric_constants`

Rework of rust-lang#10997

- uses diagnostic items
- does not lint imports of the float modules (`use std::f32`)
- does not lint usage of float constants that look like `f32::MIN`

I chose to make the float changes because the following pattern is actually pretty useful
```rust
use std::f32;
let omega = freq * 2 * f32::consts::PI;
```
and the float modules are not TBD-deprecated like the integer modules.

Closes rust-lang#10995

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changelog: New lint [`legacy_numeric_constants`]
[rust-lang#12312](rust-lang/rust-clippy#12312)