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