Common facts / checks

[clementd-fretlink]

Some patterns are common (eg ACLs, with resource-based filtering, or TTLs). The default symbol table reflects that, as it embeds symbols related to acls (right, resource) and ttls (time).
Since the spec already carries that, it would be good to explicitly mention these patterns in the spec, so that implementations can provide high-level helpers for them (the rust impl did that at some point, but the helpers were mixed with lower-level functions).

It would also be useful for biscuit-cli (a --ttl '1 day' option for instance)