PoC for Hy evaluation sandboxing

Currently it's possible to put all kinds of awful things into a WITCH macro. you can lock up the server or kill it.

This issue covers a PoC with the following acceptance criteria:

[x] WITCH scripts can't DoS the server
[x] WITCH scripts can't crash the server
[x] WITCH scripts can't divulge private information from the tildemush database or in-memory stores

Evaluate process-level sandboxing (which might be constrained by IPC issues) and syntax-level sandboxing.

vilmibm / tildemush