TomWeaver18 - refactor safe eval

[TomWeaver18]

Greetings fellow DexTerians,

I present another refactor patch, this time focusing on _safe_eval which was, imo, a bit of a mess.

I've heavily refactored, adding a new class to handle safe evaluations along with 2 new exceptions that are used internally in the new SafeEvaluator class.

all previous unit tests still pass, lint and pylama show no complaints but I am using an up-to-date version of YAPF that heavily modified 8/9 other files, so this step has been omitted to avoid pollution of the change.

happy reviewing Tom

[jmorse]

LGTM, that functionality can now actually be understood, which is great.

• Some boilerplate, but readable & understandable now
• Might be better off as a module only, but that's bike-sheddable
• Horray docstrings!

[TomWeaver18]

Safe eval is being removed by a later patch.

RIP the patch that could, but never would!