Enable Clippy use_self lint in spirv-std

[nipunG314]

As mentioned in #34, the use_self lint is currently disabled in spirv-std.

There is one exception to this. I had to introduce an exception in mod.rs because there is bug that confuses core::intrinsics with f32. I have opened a bug report for it over on the Clippy repo here. It seems there a few related errors where this also happens.

#[allow(clippy::use_self)]
impl MathExt for f32 {
    fn pow(self, factor: f32) -> f32 {
    ...
}

I'm not sure whether enabling the use_self lint is a good idea or not. The feature doesn't seem to be bug-free.

[khyperia]

Thanks so much! 🎉