der: derive(Sequence) with `heapless::Vec<u8, N>` field

[dishmaker]

https://docs.rs/tinyvec/latest/tinyvec/struct.ArrayVec.html

Having stack-allocated vec fields would be nice, maybe gated behind "tinyvec" or "heapless" feature flag.

Use case:

/// EU Tachograph G2 certificate public key
#[derive(Sequence)]
#[asn1(tag_mode = "IMPLICIT")]
pub struct CertificatePublicKeyRaw {
    pub domain_parameters: ObjectIdentifier,

    /// Public point is '04 xx .. xx   yy .. yy'
    ///
    /// Maximum: 1 + 2 * 66 bytes for Nist521
    #[asn1(context_specific = "6", type = "OCTET STRING")]
    pub public_point: ArrayVec<[u8; 133]>,
}

[tarcieri]

That would be OK as an optional dependency. We also support heapless::Vec in the aead crate