Add minetest.copy_table(table) To get rid off the "table references"

Signed-off-by: Craig Robbins <kde.psych@gmail.com>

doc/lua_api.txt

@@ -1310,6 +1310,8 @@ minetest.is_yes(arg)
 ^ returns whether arg can be interpreted as yes
 minetest.get_us_time()
 ^ returns time with microsecond precision
+table.copy(table) -> table
+^ returns a deep copy of a table
 
 minetest namespace reference
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