pyke/mapping.c, key.c: Make the mapping code more intrusive and complete.

pyke/mapping.c, key.c: Make the mapping code more intrusive and complete.

Previously, mapping classes would implement a minimum of Python-level
operations, such as iterating over keys and mapping lookups, and the
generic mapping code would just synthesize missing operations in terms
of these.

Instead, introduce a mapping-specific vtable pointer in object
structures which describes the mapping operations in a form similar to
mLib's `sym' interface, which is rather more efficient in terms of hash
probes, and have the generic mapping code synthesize all of the Python-
level operations from these.

Needless to say, this is very messy.  Sorry.