* struct.c: #include "alist.h", "weaks.h", "hashtab.h";

* struct.c: #include "alist.h", "weaks.h", "hashtab.h";
(scm_struct_table): Weak key table with auxilliary information for
struct types.  Currently used for names and wrapper classes.
(scm_struct_ihashq): Hash function for structs.
(scm_struct_create_handle): Get/create entry in scm_struct_table.
(scm_struct_vtable_name, scm_set_struct_vtable_name_x): Procedures
for accessing names of vtables.  The record implementation in
boot-9.scm currently uses the setter to record the name of record
types.  When the object system is initialized, it can use this
information to create wrapper classes with suitable names.
(scm_init_struct): Allocate scm_struct_table.
(scm_alloc_struct): Don't initialize scm_struct_i_tag here.
(struct tags are a finite resource and we might want to restrict
the use of tags to vtables only.  E.g., Goops only uses tags for
classes.)
(scm_make_struct): Use scm_struct_entity_n_extra_words instead of
magic number 5.
(scm_struct_vtable_tag): Use scm_struct_i_tag instead of magic
number -1.