Add "Rules" function on the end of "Subtype Indication"

The current (well, VHDL 2008 LRM I have access to) declares the subtype_indication on p65 to be: subtype_indication ::= [ resolution_indication ] type_mark [ constraint ]

Im proposing this is changed to

subtype_indication ::= [ resolution_indication ] type_mark [ constraint ] [rules_indication]

Where rules_indication is a function call when an object is created using the specified subtype with the following signature:

function <rules> (o : <base_t>) return <base_t>;

where o is the object declared

using my usual AXIS example (and also would be nice when paired with #79):

type axis_t is private record 
  tdata : std_logic_vector;
  tkeep : std_logic_vector;
end record axis_t;

and a user could declare a "rules" function like this:

function axis_rules(o : axis_t) return axis_t is
begin
  assert tdata'length rem 8 = 0 report "tdata length must be a multiple of 8 bits" severity ERROR;
  assert tkeep'length = tdata'length/8  report "tkeep must have 1 bit per tdata byte" severity ERROR;
  
  return o;
end function;

subtype axis_checked_t is axis_t axis_rules;

Then, as another interesting effect, you could override the initial value for a given subtype:

function axis_init(o : axis_t) return axis_t is
  variable r : o'subtype := o;
begin
  r.tkeep := (others => '0');
  return r;
end function;

subtype axis_tkeep_init_t is axis_t axis_init;

and finally, if you remove the subtype restriction on protected types, you can give yourself a "constructor"

type p_t is private protected   -- cannot create object directly
  procedure init;
end prootected p_t;

function p_t_init(o : p_t) return p_t is
  variable r : p_t;
begin
  r.init;
  return r;
end function;

subtype p_constructor_t is p_t p_t_init;

shared variable some_inited_object : p_constructor_t;   -- will create the object and call the "init" constructor