struct interface

[Dirk Herrmann <dirk at ida dot ing dot tu-bs dot de>]

Hi!

Similar as with cells, struct data memory can also hold raw data and
scheme objects.  I am currently changing the data part from SCM[] to
scm_bits_t, thus following the paradigm that SCM should only be used if
we are sure that the content is a valid scheme object.

However, as with cells, it would make sense to provide low-level macros to
read struct slots as raw data as well as scheme objects in order to reduce
the amount of packing/unpacking on the user side.

Thus, I suggest the following macros:

SCM_STRUCT_WORD(s,n)
  --> deliver the user field number n of struct s as raw data
      This will translate into:
      SCM_STRUCT_DATA(s)[n]
SCM_STRUCT_OBJECT(s,n)
  --> deliver the user field number n of struct s as a scheme object
      This will translate into:
      SCM_PACK (SCM_STRUCT_DATA(s)[n])
SCM_SET_STRUCT_WORD(s,n,v)
  --> set the user field number n of struct s as raw data
      This will translate into:
      (SCM_STRUCT_DATA(s)[n] = v)
SCM_SET_STRUCT_OBJECT(s,n,v)
  --> set the user field number n of struct s as a scheme object
      This will translate into:
      (SCM_STRUCT_DATA(s)[n] = SCM_UNPACK (v))

We could also provide the corresponding macros a la SCM_STRUCT_WORD_0, but 
I'm not sure if this is necessary.

Best regards
Dirk Herrmann