Fix search_varfb_instance_decl to be able to find variables and FB when only given an identifier.
Fix handling of FB calls in stage3.
Start handling of function invocations in stage3.

STD_TEST.st
STD_RESSOURCE.c
LOCATED_VARIABLES.h
POUS.c
test
STD_CONF.c
SFC_TEST.st
test.exe
plc.st
POUS.h
VARIABLES.csv