[m-rev.] for post-commit review: convert more passes to use var_tables

[Zoltan Somogyi]

Most of this is boring and does not need review, but I would like
Peter to take a look at the new XXXs in tupling.m, in candidate_headvars_of_proc_2.
Was the test that excluded unnamed variables from being put into tuples
intentional, and if so, what was the reasoning behind it?

Zoltan.
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