Add in support for using #[default] and #[num_enum(default)] or #[num_enum(catch_all)]

[urkle]

This resolves #165

This allows for having a default std derived Default than the fallback that num_enum uses for FromPrimitive

use num_enum::FromPrimitive;

#[derive(Default, FromPrimitive)]
#[repr(u8)]
enum MyFallbackEnum {
    #[default]
    Zero,
    #[num_enum(default)]
    NonZero,
}

assert_eq!(MyFallbackEnum::Zero, MyFallbackEnum::default());
assert_eq!(MyFallbackEnum::NonZero, 5u8.into());

#[derive(Default, FromPrimitive)]
#[repr(u8)]
enum MyCatchAllEnum {
    #[default]
    Zero,
    #[num_enum(catch_all)]
    NonZero(u8),
}

assert_eq!(MyFallbackEnum::Zero, MyFallbackEnum::default());
assert_eq!(MyFallbackEnum::NonZero(5u8), 5u8.into());

[urkle]

@illicitonion if you want to do a breaking change for a 0.8.0 release we can

1. drop ::num_enum::Default (not needed anymore IMO)
2. rename #[num_enum(default] to #[num_enum(fallback)], and drop accepting #[default] as that is more clear as to the purpose and would reduce confusion with std #[default]. (or we emit warnings when those are used to allow a migration path)