Enable use of `piqi` in `$PATH`

This enables usage of `piqi` in the most simple case: when it is already in `$PATH`.

`piqi` path is calculated as follows, in descending priority:
1. `$PIQI` environment variable.
2. `which piqi`
3. lib dir
4. deps dir

This pull request is a request for discussion. I still do not think `get-piqi-binary` is the right approach. We should always let Erlang decide which piqi binary to use (following the order above). Also, because that functionality is already in `priv/bin/piqi`.

Can you elaborate why `get-piqi-binary` is needed?