wip: datalog foreign function interface prototype

[divarvel]

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).

Open questions:

• should function names be interned as well? they are currently not

Code coverage should be improved by adding samples

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #228 will not alter performance

_{Comparing ffi (99c0718) with v5 (e59f41d)}

Summary

✅ 12 untouched benchmarks

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.30120% with 82 lines in your changes missing coverage. Please review.

Project coverage is 69.58%. Comparing base (3930767) to head (99c0718). Report is 1 commits behind head on v5.

Files with missing lines	Patch %	Lines
biscuit-parser/src/builder.rs	0.00%	34 Missing :warning:
biscuit-auth/src/token/builder.rs	86.76%	18 Missing :warning:
biscuit-auth/src/format/convert.rs	74.13%	15 Missing :warning:
biscuit-auth/src/token/authorizer.rs	50.00%	8 Missing :warning:
biscuit-auth/src/datalog/expression.rs	90.90%	4 Missing :warning:
biscuit-auth/examples/testcases.rs	92.85%	2 Missing :warning:
biscuit-auth/src/token/authorizer/snapshot.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v5 #228 +/- ## ========================================== + Coverage 69.03% 69.58% +0.54% ========================================== Files 25 25 Lines 5926 6154 +228 ========================================== + Hits 4091 4282 +191 - Misses 1835 1872 +37 ```

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