Reorganize khash decl macros to separate typedef

Reorganize khash decl macros to separate typedef

This separates the khash declaration macros into three pieces: one
to declare the type, one for the function prototypes, and one for
the function declarations.  This makes it easier to instantiate a
khash type one time and have a header with just the typedef and
prototypes.

The old macros are redefined in terms of the new ones.