Add fold and vfilter entry-points to Ho_Net module

The second is called vfilter because it's only filtering on the stored
value, not the "key" (the term list*term pattern). Similarly, the
fold only gets access to the stored values.

Remove TABs in some source code, replacing them with spaces

Use Emacs's conception of how many spaces the TAB was
representing (and the M-x untabify command).

This may not line up with the author's original conception, but then,
using TABs makes it impossible to tell just what the original
conception was in the first place anyway.

Should probably script this over all of repository (excluding
makefiles of course), and use standard expand or col utilities rather
than emacs.

rename Ho_Net.empty_net to Ho_Net.empty

All similar structures (e.g. Net and Raw) use "empty" not "empty_net";
it seems right to follow a single naming convention.

Change src/bool to src/1 as a prelude to experimentation!