#770 PLCgen: Types in expressions, check them.

• Changes:
  • Adds a function block type to use a variable type.
  • Function block description knows the type name, and takes an abstract result type (like regular function descriptions)
  • Function block variables are just normal variables now
  • Function block applications take the variable as that has all the needed data.
  • Compute the concrete type of a function application or function block application. All expression classes now have type, thus dropping the null type cases.
  • Fix errors in the function definitions.
  • Cast expression uses the type of the child expression.
  • Some tests are impossible now due to the new type restrictions
  • Types in timer handling were a mess, fixed it.
  • Check types in assignment and guards of an IF statement. (Didn't cause new problems.)
  • Update test expectations for the modified timer code.

The only end user visible change is that this fixes timers.

Adresses #770 (closed)