Cross-platform subprocess support

[talex5]

This is based on @patricoferris's #473, but rebased on top of #498 and with some changes:

• The standard streams are now optional, and default to inheriting from the parent.
• The executable is optional, and searched for in $PATH from the first argument if not given.
• Process.parse_out provides an easy way to parse the output of a process.
• The logic for converting flows to pipes is now shared in Eio_unix.

This also splits Eio_unix into separate modules.

[patricoferris]

Thanks! This looks great, I like the parse_out API.