[m-rev.] for post-commit review: significantly improve psqueue.m

[Zoltan Somogyi]

This diff improves so many things that it is almost a rewrite,
which is why I am attaching the new version of psqueue.m.
The algorithms (with a few exceptions) stay the same, but many of
their implementation details have been changed.

For review by anyone interested.

Zoltan.
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