esc: make traces optional

[pgavlin]

And add a couple of helper methods: one to remove trace metadata from values and one to remove all metadata from an Environment. This allows embedders to more easily provide less heavyweight representations of evaluated values.

[pgavlin]

Part of the motivation here is the potential to add support in the ESC API for returning an opened environment in a form that does not include this metadata. The metadata-free form is likely to be easier to consume for most API-level users.

[borisschlosser]

lgtm! Let me know your thoughts where we can leverage these changes in ESC.

[nyobe]

I think rather than stripping metadata, it might be nice to have a helper that converts an esc.Value tree into a go value. I think users asking for a simpler representation probably just want a direct mapping to the shape the environment defines.

Then we don't lossily drop metadata, we just provide a convenience mapping for direct lookups when desired.

Edit: actually we already have this with the ToJSON() helper!

[pgavlin]

Edit: actually we already have this with the ToJSON() helper!

Yes--we already have ToJSON. This is intended to allow us to do something similar to ToJSON at the REST API level but maintain secret-ness metadata.

It could be that we should just add a parameter to the REST API that allows all metadata to be discarded (i.e. call ToJSON and return the result). I just don't love dropping the secret-ness.