impl TryFrom<vcard4::Vcard> for VCard4

[RemiBardon]

To parse text/vcard data into VCard4 types, the Prose Pod API is using tmpfs/vcard4. To simplify conversions, we could move the mapping function here as a TryInto. This would also help us notice missing RFC6350 keys (see #102).

Note 1: This should be behind a feature flag.

Note 2: I might do it myself at some point, I just created an issue here so we don't forget.