feat: add inherit circuit tag

[ivokub]

Resolves #385.

This PR adds new "inherit" tag to the circuit which allows inherit the visibility of the parent object. Useful for custom defined types.

Meanwhile, updated also some comments to take advantage of new go doc formatting rules (links and lists).

[ivokub]

There was before some inconsistency how the tags were parsed. If there was not tag, then we set the visibility from the parent (https://github.com/ConsenSys/gnark/blob/develop/frontend/schema/schema.go#L256 and https://github.com/ConsenSys/gnark/blob/develop/frontend/schema/schema.go#L281). If there was only a name tag gnark:"X", then we set the visibility to secret (https://github.com/ConsenSys/gnark/blob/develop/frontend/schema/schema.go#L264). The tests didn't have the edge case.

I'm trying to make the behaviour consistent and keep the existing promise. So, on top-level if there is no tag, then the visibility is left to Unset, but this resolves (even if the top-level element has sub-elements) in the variable parser to Secret. In every other case when there is not tag, we assume the visibility of the parent.

If there is only name tag, then for top-level variables we assume Secret visibility and lower levels inherit from the parent (this is the difference, previously we set the visibility to Secret).

Added also explicit "inherit" tag option which is like empty one, but which can not be set for the top-level element. I made this change before figuring out the name-only tag cases and now seems to be slightly redundant. But dunno, maybe is useful.

Question -- shouldn't we in the case when there is not tag and we are top-level, set the visibility to Secret eagerly? E.g. when we hit the case https://github.com/ConsenSys/gnark/blob/develop/frontend/schema/schema.go#L280. It should work now also nicely as it expected, but we take slightly different paths (compared to when we have name-only tag) and this could at some point create inconsistent behaviour.