Add support for generic type constraints. They are currently only useful to stop specialization of a generic. The parsing of a generic is still too lax and needs to be changed in the future...

symconst.pas:
  + extend "tdefoptions" by a "df_genconstraint" which will be used to mark dummy defs that should mainly satisfy the compiler's type checking without to much changes
symsym.pas:
  + add a class "tgenericconstraintdata" which will hold information about the constraints associated with a specific generic type parameter (designed for future extensions)
  + extend "ttypesym" by a reference to a "tgenericconstraintdata" which is written to the ppu only if needed
symtype.pas:
  + add a pointer to "tderef" as this is needed for the ppu reading/writing code for the "tgenericconstraintdata"
pdecl.pas, types_dec:
  + call "parse_generic_parameters" so that constraints are allowed
pgenutil.pas:
  + extend "generate_specialization" by a "parsedpos" to give in the file position of the first parsed parameter (neede...