Apply couple of rule of thumbs to simplify highlighting code

Main one: instead of adding a parameter to function to handle special
case, make the caller handle it.

Second main one: make sure that function does a reasonable thing.
`highlight_def` picks  a color for def, *regardless* of the context
the def is use. Feeding an info from the call-site muddies the
responsibilities here.

Minor smells, flagging the function as having space for improvement in
the first place:

* many parameters, some of which are set as constants on most
call-sites (introduce severalfunction instad)
* boolean param (add two functions instead)

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