Extra execution restrictions

[divarvel]

Following discussions in #50 and #56, i've added extra (configurable) limitations to the haskell implementation: https://hackage.haskell.org/package/biscuit-haskell-0.1.0.0/docs/Auth-Biscuit-Datalog-Executor.html#t:Limits

• allowRegexes: arbitrary regexes are a well known DoS vector. While specifying the regular expression features #50 tries to tackle the (ambitious) goal of defining a regex subset that would ensure runtime characteristics, an effective solution is to completely disallow regex computation.
  The haskell impl mitigates a lot of the regex issue with how the timeout is implemented (verification is forked to another green thread), but the rust implementation has a blocking verification, so a redos would not be prevented by the timeout feature.
• allowBlockFacts: Attenuating a biscuit with anything other than checks is inherently complex and error-prone, the #authority / #ambient restriction can be hard to understand (and was indeed the source of a security issue). A solution is to completely prevent blocks from defining facts or rules. A better solution would be to store this info in the token itself, but as a first step, having a simple way to forbid it in the verification phase should be enough.