Code and interface refactoring
Add basic validation of the .piq-alias field and option property
Minimize dependency of piqi on piqi-ocaml and vice a versa

This is done in preparation fo moving piqi-ocaml into a separate repo.
Improve build scripts

- automatically build piqilib when building piqi
- automatically re-build piqi on piqilib changes
Add support for a record-level .piq-positional setting

When specified, it applies to all record fields unless overridden by a per-field
setting.
Check for infinite types
Move some code between piqilib and piqic-ocaml

Remove piqilib/piqi_graph.ml and add its functionality to piqic-ocaml as it was
the only caller of the Piqi_graph functions.