Add a `secureEvaluate` API to snap environment

[danfinlay]

The snap execution environment already uses SES, so we could easily provide snaps the ability to securely execute scripts within their own environment.

These scripts would have the same global API by default as snaps (not much, but whatever we deem globally safe), and it would allow for easy/safe (as safe as the snap system itself) extensibility within the snap ecosystem.