Add infrastructure for generating examples for many filters

* tools/pflang-compile: Rewrite to just use the modules instead of
  subprocesses.

* tools/dump-markdown: New tool.

* src/pf/codegen.lua (compile_lua): Remove needless "dlt" usage.

* src/pf.lua (compile_filter): If the "source" option is present, return
  source.

* doc/Makefile:
* doc/tcp-port-80.md: Arrange to generate markdown examples using the
  new dump-markdown script.

* README.md: Remove examples, rework some other text.
More examples

* doc/README.md: Add index.

* doc/Makefile:
* doc/icmp-or-tcp-or-udp.md: New example.
Remove helpers/pflang_to_lua.lua

* tools/helpers/pflang_to_lua.lua: Remove; no longer needed.