Opaque foreign values

[jneem]

This is some kind of baby step towards FFI: allow for opaque foreign values to be injected into nickel and copied around, but not otherwise interacted with or modified from within nickel.

This was a request from the keystone project, with some discussion on the discord.

Question: what should be payload of the opaque foreign values be? u64 might be sufficient: that gives a lot of values, and the user could "embed" richer data by maintaining a lookup table on their side. Maybe Box<dyn Any> is also a possibility?