Compute rule signature at rule initialization to save perf

[benoit74]

Fix #370

Changes:

• use a generic class to automatically compute function signature at rule initialization
• use cached value at "runtime"

[rgaudin]

I'd like us to discuss alternatives as I find this optimization very intrusive. Haven't dug much into the issue but it looks like what lru_cache is for at first glance.

[benoit74]

Replaced by #375