refactor: save a function call on non-table environment recursion.

This shaves over 4% off the time of Specl's make check: from 27.2
to 26.0 seconds averaged over 3 runs.
* lib/specl/util.lua (deepcopy): Only recurse when a whole table
needs to be copied, otherwise deal with already-copied tables and
non-tables in-situ.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>