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.
Things that we must still check for in stage 3:
1) Handling of CONSTANTs:
1.a) "Any program organization unit attempts to modify the value of a variable that has been declared with the CONSTANT qualifier;"
1.b) From table 16.a "The CONSTANT qualifier shall not be used in the declaration of function block instances as described in 2.5.2.1."