add AsRef impl for Record to allow for passing records by references when signing

[hunts]

One of the use case is to allow for both owned or borrowed records in the mutable slice when composing the signed data.

Without this, in some cases, the caller would have to clone records if they only hold a (im)mutable slice of record references or an immutable slice of record values.

[partim]

Thank you for the PR! This looks like a useful change, indeed.

[hunts]

Thank you Martin, cheers!