Remove unnecessary deepcopies

[moradology]

Calls to deepcopy are fairly expensive when working with large number of STACs. In to_dict methods, a dictionary d is instantiated only to have deepcopy applied to it for the return value. This seems to be unnecessary.

Here's snakeviz output of some cProfile timings.
Before:

After:

It should be noted that the after includes also the use of orjson to optimize ser/de operations. This optimization can be left out of pystac because it requires an external dependency and because it can easily be plugged in through a STAC_IO subclass

[matthewhanson]

nice catch @moradology , this looks good to me, there doesn't seem to be any reason to do a deepcopy of a locally defined variable before returning,
I'm also interested in improving the performance of PySTAC with large catalogs and interested in any optimizations you've tried.