der: derive(Sequence) with [u8; N] field

[dishmaker]

[u8; N] arrays should be allowed as OCTET STRING fields.

#[derive(Sequence)]
pub struct MySequence {
    #[asn1(context_specific = "2", type = "OCTET STRING")]
    pub authority_reference: [u8; 8],
}

error[E0277]: the trait bound `der::asn1::OctetStringRef<'_>: TryFrom<[u8; 8]>` is not satisfied
  --> example.rs:68:10
   |
68 | #[derive(Sequence)]
   |          ^^^^^^^^ the trait `From<[u8; 8]>` is not implemented for `der::asn1::OctetStringRef<'_>`
   |
   = help: the following other types implement trait `From<T>`:
             `der::asn1::OctetStringRef<'_>` implements `From<&der::asn1::OctetString>`
             `der::asn1::OctetStringRef<'_>` implements `From<&der::asn1::OctetStringRef<'_>>`
   = note: required for `[u8; 8]` to implement `Into<der::asn1::OctetStringRef<'_>>`
   = note: required for `der::asn1::OctetStringRef<'_>` to implement `TryFrom<[u8; 8]>`
   = note: this error originates in the derive macro `Sequence`

does not work with deref ="true" too.

the trait `From<&[u8; 8]>` is not implemented for `OctetStringRef<'_>`

[Spoiler] real use case

/// EU Tachograph certificate body
#[derive(Sequence, Debug, Clone)]
#[asn1(tag_mode = "IMPLICIT")]

pub struct TachographCertificateBodyRaw {
    /// primitive, 1 byte
    #[asn1(application = "41", type = "OCTET STRING")]
    pub profile_identifier: [u8; 1],

    /// primitive, 8 bytes
    #[asn1(application = "2", type = "OCTET STRING")]
    pub authority_reference: [u8; 8],

    /// primitive, 7 bytes
    #[asn1(application = "76", type = "OCTET STRING")]
    pub holder_authorisation: [u8; 7],

    /// constructed
    #[asn1(application = "73")]
    pub public_key: CertificatePublicKeyRaw,

    /// primitive, 8 bytes
    #[asn1(application = "32", type = "OCTET STRING")]
    pub holder_reference: [u8; 8],

    /// primitive, 4 bytes
    #[asn1(application = "37", type = "OCTET STRING")]
    pub effective_date: [u8; 4],

    /// primitive, 4 bytes
    #[asn1(application = "36", type = "OCTET STRING")]
    pub expiration_date: [u8; 4],
}

[tarcieri]

I think this just needs TryFrom impls to convert OctetStringRef to [u8; N]