Keep hidden fields in std.prune

[ricardbejarano]

According to the docs, std.prune:

Recursively remove all "empty" members of a. "Empty" is defined as zero length arrays, zero length objects, or null values. The argument a may have any type.

However, it also removes hidden fields from objects because it uses std.objectFields to traverse it. This PR changes that to use std.objectFieldsAll instead, which keeps hidden fields as promised by the docs.

Sort of relates to #1167.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[ricardbejarano]

CLA signed!

[CertainLach]

This way, every hidden field will be unhidden, and this will be a breaking change.

Also, fields referencing self/super will be frozen, and I don't think this is an useful thing after that?

[ricardbejarano]

What about these other options?

1. std.prune(a, keep_hidden=false)
2. std.prune(a, arrays=true, objects=true, nulls=true, hidden=true)
3. std.pruneEx(a, arrays=true, objects=true, nulls=true, hidden=true)

Showing the default-state function headers here, users would then be able to override whatever they want to be pruned or not.