Add get_array method to Buf

[lperlaki]

This would increase the MSRV to 1.51 since it uses const generics!

I am not sure on the MSRV policy of bytes, so if this is a problem we can just close this PR

Added Methods

• Buf::try_get_array
  is a copy of the old implementation of buf_try_get_impl
• Buf::get_array simply calls try_get_array and panic_advance on error

the second commit changes the implementation of the get_x methods to use try_get_array

this would also enable Buf implementors to optimise (like #622) the get_x methods by overriding try_get_array

[lperlaki]

as related work there is also a std lib proposal to add a std::io::Read::read_array rust-lang/rust#148848

[Darksonn]

I don't really get why we should add this. It seems pretty niche.

[lperlaki]

I find the method useful for decoding data from a Buf

let mut key_buf = [0u8; 32];
buf.copy_to_slice(&mut key);
let key = key_buf;

compared to

let key: [u8; 32] = buf.get_array();

this can also be achieved with an extension trait

pub trait BufExtArray: Buf {
     fn get_array<const N: usize>(&mut self) -> [u8; N] {
        self.try_get_array()
            .unwrap_or_else(|error| panic_advance(&error))
    }

    fn try_get_array<const N: usize>(&mut self) -> Result<[u8; N], bytes::TryGetError> {
        if self.remaining() < N {
            return Err(bytes::TryGetError {
                requested: N,
                available: self.remaining(),
            });
        }

        let ret = self.chunk().first_chunk::<N>().copied();

        if let Some(ret) = ret {
            // if the direct get was possible, advance and return
            self.advance(N);
            Ok(ret)
        } else {
            // if not we copy the bytes in a temp buffer
            let mut buf = [0; N];
            self.copy_to_slice(&mut buf); // (do the advance)
            Ok(buf)
        }
    }
}

impl<B: Buf + ?Sized> BufExtArray for B {}

so if you find it to niche for including please close this PR.