A structure is a first class data type which holds Scheme values
or C words in fields numbered 0 upwards. A vtable represents a
structure type, giving field types and permissions, and an optional
print function for write etc.
Structures are lower level than records (see Records) but have
some extra features. The vtable system allows sets of types be
constructed, with class data. The uninterpreted words can
inter-operate with C code, allowing arbitrary pointers or other values
to be stored along side usual Scheme SCM values.