[m-rev.] for review: optimize the tests for builtin types when printing terms

[Zoltan Somogyi]

For review by Julien, since it was his mod of term_to_xml.m that
tickled my memory of this old todo item.

On my laptop, this diff speeds up the attached microbenchmark
from about 2.6 to about 1.6 seconds.

Zoltan.
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