Reduce the number of strlen calls in the jansson backend.
Test on my laptop (count of the instructions)
Before:
❯ valgrind --tool=callgrind ./bin/test_object
==3105045== Events : Ir
==3105045== Collected : 441453
After:
❯ valgrind --tool=callgrind ./bin/test_object
==3144451== Events : Ir
==3144451== Collected : 440597
Signed-off-by: Maxim Zhukov <mussitantesmortem@gmail.com>