Detect usage of custom floating-point abs implementation

[krishna-veerareddy]

Detect usage of

if a < 0.0 { // or a.is_sign_negative()
    -a
} else {
    a
}

and

if a > 0.0 { // or a.is_sign_positive()
    a
} else {
    -a
}

and suggest usage of abs instead

a.abs()

Using abs is more efficient, handles more cases and is more succinct than using the if/else block so this lint could go under the suboptimal_flops lint. If this seems like an obvious thing and isn't worth the check then I'd be happy to close it.

[flip1995]

This should also lint the opposite case:

if a < 0.0 { // or a.is_sign_negative()
    a
} else {
    -a
}

and suggest -a.abs()

[JarredAllen]

I can work on this. This seems like a good point to begin contributing.

[flip1995]

Thanks! If you have questions, just ask here or open a WIP PR. A good read for starting writing lints is this documentation: https://github.com/rust-lang/rust-clippy/blob/master/doc/adding_lints.md