const fn for `StatusCode::as_u16`

[y-haidar]

Are there plans for this? Would like to use it in constant generics.

https://github.com/hyperium/http/blob/bda93204b3da1a776cf471ed39e8e374cec652e7/src/status.rs#L119-L121

Change to this:

    pub const fn as_u16(&self) -> u16 {
        (*self).0.get()
    }