Macro deku_error 

macro_rules! deku_error {
    ($p:path, $desc:expr, $fmt:expr, $($arg:expr),*) => { ... };
    ($p:path, $desc:expr, $fmt:expr) => { ... };
    ($p:path, $desc:expr) => { ... };
    ($p:path) => { ... };
}

Abstract over alloc vs no-alloc for handling of error strings

The macro takes the DekuError variant as an argument and performs the instantiation itself in order to cater to the no-alloc case where we discard all arguments beyond the description.

Invoked like:

use deku::{deku_error, DekuError};
let _ = deku_error!(DekuError::Parse,
                    "bit size is greater than input size",
                    "{} exceeds {}",
                    bit_size,
                    input_size);