wip: datalog foreign function interface prototype

This allows using external functions in datalog.

This makes it easy to provide custom logic without extending the spec for every use-case, at the expense of portability: behaviour is no longer guaranteed to be consistent cross languages, and some languages won’t be able to support it at all (for instance JS as of now).

Todo:
- stricter conversions from datalog
- feature-gating if possible

Open questions:
- enum index for the FFI variants (contiguous or not?)
- how to provide functions (right now, function pointers: prevent mutability and closing over arguments)
- how to provide arguments (right now, datalog::Term, so symbols have to be resolved, and functions returning strings have to register new symbols)

biscuit-auth/src/capi.rs

@@ -79,6 +79,7 @@ pub enum ErrorKind {
  FormatPublicKeyTableOverlap,
  FormatUnknownExternalKey,
  FormatUnknownSymbol,
+ FormatMissingFfiName,
  AppendOnSealed,
  LogicInvalidBlockRule,
  LogicUnauthorized,
@@ -158,6 +159,7 @@ pub extern "C" fn error_kind() -> ErrorKind {
  ErrorKind::FormatUnknownExternalKey
  }
  Token::Format(Format::UnknownSymbol(_)) => ErrorKind::FormatUnknownSymbol,
+ Token::Format(Format::MissingFfiName) => ErrorKind::FormatMissingFfiName,
  Token::AppendOnSealed => ErrorKind::AppendOnSealed,
  Token::AlreadySealed => ErrorKind::AlreadySealed,
  Token::Language(_) => ErrorKind::LanguageError,