Add vcd::all_long_names()

The idea here is that pretty much the whole point of vcddiff is to
produce this list of names.  This tool allows users to explicitly walk
the list.
Add vcdsigs binary

This allows command-line users access to the just added
vcd::all_long_names() method.  The idea is that when your vcddiff
bails out and says there are mis-matched signals that you can now
debug exactly why that happens!