[PoC] Implement parser for permissions framework

[GPaoloni]

Description

This PR proposes a different approach to #520.
Here, I exemplify how we might use parser combinators to parse the expressions and map them to functions. This functions are in turn evaluated against user and target for each action as usual.

The advantage of moving to an approach like this is that we can simplify the expressions in the permissions frameworks with and with relatively little effort change from expressions like
"editContact": [["isSupervisor"], ["isOwner", { "createdHoursAgo": 24 }]]
to an analogous like
"editContact": "isSupervisor OR (isOwner AND createdHoursAgo(24))".

NOTES:

• This PR does not means that we want/should remove the way we are currently evaluating the JSON-represented permissions rules. Instead, we could combine both approaches to use a more human readable DSL that is parsed and converted into the JSON-represented permissions rules.
• This approach makes it easier to "validate" a JSON definition file IMO, as the rules for what is valid in the language are embedded in the parser, i.e. if a file parses then it's valid.