Add datatype checking of enumeration data type declarations.

Print const values of symbol.

Fix code generation of enum value constants (this is a temporary fix - it will be followed by a code cleanup).

Fix bug in datatype checking of enumerated datatypes delcarations.

Fix datatype checking of enumerated datatype declarations. We now follow the fill->narrow algorithm correctly.

Add method to determine the id (name) of a datatype.

Change the base datatype of enumerations to an enumerated_type_declaration_c (for named types!)

Change the public interface to debug functions.

Check for errors in enum datatype declarations. (This is not yet complete as it will not yet stop the compilation process if these errors are found)

Fix one more bug - upper_bound() works a little differently to what we want!

Fix more bugs in dsymtable_c (real semantics of std::multimap::lower_bound is not what was assumed!)

Fix counting bug in dsymbtable_c

Use duplicate symtable instead of symtable for enum constant value table (this will later allow us to detect semantic errors in IEC 61131-3 source code)

Move the enumerated_value_symtable to fill_candidate_datatypes_c, since it is only used there!