The name "new" happens to be a C++ keyword, which was the my reason to
rethink those names.
The "xlat_table" is not only used to translate pointers for persisting
scripts to shared memory, but is also used to annoate pointers
(e.g. by the JIT to associate an op_array with its jit_extension).
The names "old" and "new" aren't good for that; often, there's nothing
"old" or "new" about them. It's actually a generic lookup table, and
"old" shall be named "key" (which it is called internally already),
and "new" is renamed to simply "value".
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