Some users of Jiffy have experienced issues when decoding large JSON
documents. Normally Jiffy expects smallish documents and returns any
strings as sub-binaries. When dealing with large documents these
sub-binary references can keep a large amount of RAM around unless the
user goes through and applies `binary:copy/1` on every string returned
from Jiffy. This however causes a large amount of CPU usage to do
something that Jiffy could do as it builds the JSON structure.

The `copy_strings` decoder option does exactly this. Instead of
returning sub-binaries Jiffy now copies every string into a newly
allocated binary. Users report that this fixes the memory issues while
also not negatively affecting performance significantly.