[plt-scheme] Exporting the struct syntax object from SRFI-4

Hi all,

I would occasionally be useful to subtype SRFI-4 vectors (to
distinguish different types of vectors).  However the current
implementation doesn't provide the structure type value needed to do
this.  It's a simple change to do so -- just provide the value after
the struct definition on line 1068 of foreign.ss, and make a few
modifications to the surrounding code to create the ID etc.  As SRFI-4
already defines a value for the name TYPE, I suggest the name s:TYPE
(e.g. s:f64vector) for the struct type.

Thanks,
Noel

PS: I can do this if no-one objects.