add new inspector function to tap current value

[behrica]

Fixes #3548

add new interactive function cider-inspector-tap-current-val to inspector.

It sends the current inspector value to the tap>.
It uses a new middleware op, inspect-tap-current-value which has been added recently to cider-nrepl.

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[vemv]

Looking good

[bbatsov]

Might be good to expand on this a bit in the debugging techniques/inspector part of the docs.

[behrica]

I added in docu a TIP to tap values from inspector in order to render them graphically using other tools.
This is at least my main use case.

[behrica]

I would have one more question on this PR.
My "initial use case" requires as well that the "tapped values" needed to be "transformed" by Clojure code before they get tapped.

This gets complicated by the fact, that the user would need to give "feedback" when this happens.

The example is "having a map deep in a value which represents 'vega-lite' data"
Clerk has a special vega-lite render, which can be "selected" by "wrapping" the value with some :meta before tapping it.
The point is that the user needed to be "asked" before tapping "what type of data" its is (= vega lite). Tihs cannot be inferred from the data as such.
Maybe this becomes more clear by look at an adhoc function I did for this purpose:
https://github.com/behrica/clerk-mode/blob/bff0aa274eae0923b6b0f3b64d32ab1fa3333c7f/clerk-mode.el#L80

We had as well a discussion on this here: #3311

Do you think the current code of this PR allows this easely, as an extension by using "advice" for example ?

[vemv]

I got these changed into 966cae4 preserving attribution.

Thanks much!

Do you think the current code of this PR allows this easely, as an extension by using "advice" for example ?

Probably not, but feel free to give it a try.

A clean solution would be to make the cider-nrepl accept a transformer function (fully-qualified symbol) - something like that?

Users could bind a specific key in the inspector map to perform cider-inspector-tap-current-val passing that extra argument.

Lastly, I dropped the path code, it turned out to excessively complicate things. It would be easier if cider-nrepl simply returned a (well-formed!) :path as part of every reponse that included a :value.