Add diagnostic::on_unimplemented attribute on rust >=1.78

[wcampbell0x2a]

• Add the following diagnostics to hopefully lead people in the right direction when seeing trait problems related to ctxs and derives

error[E0277]: the trait bound `FieldF: deku::DekuReader<'_, _>` is not satisfied
  --> examples/example.rs:37:14
   |
37 |     field_f: FieldF,
   |              ^^^^^^ the trait `deku::DekuReader<'_, _>` is not implemented for `FieldF`
   |
   = note: implement by adding #[derive(DekuRead)] to `FieldF`
   = note: make sure the `ctx` sent into the function matches `FieldF`'s `ctx`
   = help: the following other types implement trait `deku::DekuReader<'a, Ctx>`:
             <() as deku::DekuReader<'_, Ctx>>
             <(A, B) as deku::DekuReader<'a, Ctx>>
             <(A, B, C) as deku::DekuReader<'a, Ctx>>
             <(A, B, C, D) as deku::DekuReader<'a, Ctx>>
             <(A, B, C, D, E) as deku::DekuReader<'a, Ctx>>
             <(A, B, C, D, E, F) as deku::DekuReader<'a, Ctx>>
             <(A, B, C, D, E, F, G) as deku::DekuReader<'a, Ctx>>
             <(A, B, C, D, E, F, G, H) as deku::DekuReader<'a, Ctx>>
           and 152 others

• Rust Release: https://blog.rust-lang.org/2024/05/02/Rust-1.78.0.html
• Rust MR: Stabilize the #[diagnostic] namespace and #[diagnostic::on_unimplemented] attribute rust-lang/rust#119888

[github-actions]

Benchmark for 140a86b

Click to view benchmark

Test	Base	PR	%
deku_read_bits	1286.8±26.22ns	1287.4±19.33ns	+0.05%
deku_read_byte	5.4±0.08ns	5.4±0.12ns	0.00%
deku_read_enum	2.6±0.07ns	2.6±0.05ns	0.00%
deku_read_vec	34.6±0.48ns	34.5±0.48ns	-0.29%
deku_write_bits	160.9±1.77ns	161.5±3.14ns	+0.37%
deku_write_byte	22.6±0.37ns	22.7±0.33ns	+0.44%
deku_write_enum	21.4±0.30ns	21.8±0.37ns	+1.87%
deku_write_vec	439.0±6.14ns	431.8±5.93ns	-1.64%