Rename and namespace `FPCategory`

Rename `FPCategory` to `FpCategory` and `Fp* to `*` in order to adhere to the
naming convention

This is a [breaking-change].

Existing code like this:
```
use std::num::{FPCategory, FPNaN};
```
should be adjusted to this:
```
use std::num::FpCategory as Fp
```

In the following code you can use the constants `Fp::Nan`, `Fp::Normal`, etc.