Move service.[ch] to service/handler.[ch]
nuts_kex_get_algorithm()
nuts_kex_activate_algorithm(), nuts_kex_get_active_algorithm()
Move nuts_kex_consume() to kex/handler.h
Merge kex_init.c into kex/handler.c
Move kex.[], kex_dh.[ch] to kex/
Remove outdated str-module