der: add indefinite length

[dishmaker]

We can use u32::MAX as indefinite variant

[tarcieri]

I've been trying to work on indefinite lengths but in a way that avoids the need to have dummy lengths or an IndefiniteLength type

[dishmaker]

I tried a trick with NonZero<u32> but really what we need is NonMax<u32>

[dishmaker]

Because

/// This enum is 4 bytes
enum LengthInner {
    Definite(NonZero<u32>),
    Indefinite,
}

[tarcieri]

@dishmaker that enum looks a lot like IndefiniteLength (which I just removed in #1807) though we still need to support definite lengths of zero

[dishmaker]

Yeah, so we need NonMax<u32> and change Length to a 4-byte enum (to allow compiler optimizations)

https://docs.rs/nonmax/latest/nonmax/struct.NonMaxU32.html

[tarcieri]

@dishmaker I definitely don't want to pull in an extra dependency for that. der is otherwise dependency-free.

The newtype can enforce its own maximum like it did before #1726