Make Scheme_hash_table just use the native hash table type. This

Make Scheme_hash_table just use the native hash table type.  This
consists of two commits:



Implement Scheme_hash_table in terms of Smob0


Introduce Smob0 class without external storage



The Smob0 class is actually even more of a joke than the Smob1 class
already and makes do without introducing an actual Smob or its own
type.  It just allows a syntactic class wrapper around an actual
Scheme data structure to provide the normal Smob C++ programming
interface.  There might be a point in trying to make for a way of
working with Scheme traits that does not go through class pointers as
a prescribed interface in the first place.  In the current framework,
this may be the cheapest way to get rid of an additional class
indirection without losing the convenience of working with a fixed C++
type and associated member functions.

[Dan Eble, 2020-05-09 14:05:41]

LGTM.  Thanks.