libflo: A C++ library for parsing Flo files

Chisel has a "flo" backend, which outputs "flo" files.  These are
essentially just representations of the internal Chisel graph format,
which probably makes it the easiest way to interface external tools
with Chisel.

This code originated while trying to write a DREAMER ISA simulator but
appears to be useful outside of just that so it's been split off into
a library.

libflo uses pconfigure as a build system
<http://github.com/palmer-dabbelt/pconfigure>.

An example of how to use libflo can be seen in src/canon.c++, which is
t he source efor a command-line program that reads in a Flo file and
then writes it back to stdout in canonical form -- this means with all
widths inferred and in dataflow order.