Sandbox-available evel.Function not correctly sanitized

[natevw]

The following code still breaks out of the sandbox:

Function.__proto__.constructor("return this")()

As surmised in #17, the fix in #15 merely shadows (rather than fully replacing) the original constructor property. Don't think this will be too hard to fix properly.

[kumavis]

Ah clever attack! and well plugged. Looks like the challenge is back on : )