"host" and "net" fail faster

* src/pf/expand.lua: For "host" and "net", eagerly dispatch on protocol
  type.

* doc/host-127.0.0.1.md: Regenerate.
add "net 127.0.0.0/8" example

* doc/Makefile:
* doc/net-127.0.0.0-8.md: New example.
Detect and merge common tails

* src/pf/optimize.lua (simplify_if): Handle more cases.  Detect and
  merge common tails.  (This might not be a good idea, as it decreases
  precision of flow analysis; dunno.)

* doc/host-127.0.0.1.md:
* doc/net-127.0.0.0-8.md: Regenerate.
Merge pull request #47 from Igalia/host-and-net-optimizations

Host and net optimizations