Secure Table.eval()

[iguinn]

Right now, Table.eval() uses python's eval function, which is considered unsecure because it can execute arbitrary python code. This issue is to discuss how we could secure it. My suggestion is that we should limit eval to use only recognized functions and variables from the Table, and maybe also some explicit provided variables by the user.

One suggestion is to compile the string into ast (https://docs.python.org/3/library/ast.html), and then use that to either verify that the code doesn't do anything we consider unsafe. This is a python standard library feature that is extremely flexible (meaning we could extend it as needed). It's also what was used for dspeed (although this feature shouldn't have to be that complicated). We could even consider modifying the code (e.g., we could replace all and any with ak.all and ak.any for ease of use).