[m-rev.] for post-commi review: unify erlang implementation of mutables

[Zoltan Somogyi]

For review by anyone. However, what is worth reviewing is not
so much this diff, as the contents of add_mutable_aux_preds.m
after its installation.

I also noticed that the code implementing mutables for the Erlang
backend assumed that trailing was not enabled. I checked, and I found
comments in the compiler that say that the Erlang backend does not
implement trailing, but I can find nothing in handle_options.m to generate
an error if users try to compile something with --grade erlang --use-trail.
Is this intentional? Peter, do you know?

Zoltan.
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