Long executing seval expressions can cause the bot to timeout

[graymalkin]

There should be some timeout on seval to prevent long-running calculations from crashing the bot. If it's easy to it'd be nice to prevent large memory explosion too which might cause the bot to be killed by the OS.

[graymalkin]

See this similar issue on Pyperbot's predecessor ellxc/piperbot#66

[graymalkin]

And this one ellxc/piperbot#61