const fn for `StatusCode::as_u16`

[y-haidar]

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

http/src/status.rs

Lines 119 to 121 in bda9320

	pub fn as_u16(&self) -> u16 {
	(*self).into()
	}

Change to this:

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

[WhyNotHugo]

See: #680