Revert engine meta values back to a map in a `Meta` field.

[bbuck]

This will require adjusting how a lua.Engine handles converting func to a lua.Value type so that that the wrapping gopher-lua compliant function uses closures to keep a reference to the actual lua.Engine. While this doesn't eliminate the need for dynamic casting it does eliminate the concerns of having data arbitrarily overwritten, needing to whitelist them, and so forth.