sync: migrate to /bin/osh

[AladW]

oil has amongst others vastly improved error handling and special data exchange formats such as QSN. Some references:

• Oil Fixes Shell's Error Handling
• Oil vs. Shell idioms
• QSN
• TSV2 Proposal

Interesting auxiliary material:

• The Simplest Explanation of Oil
• Parsing Bash is Undecidable
• UXY: Adding structure to the UNIX tools

General documentation: https://github.com/oilshell/oil/tree/master/doc

[AladW]

The current approach in aur-sync is to take complicated parts and isolate them to helpers (using the perl AUR library, e.g. aur-sync--filter.) I think this is a more robust approach than trying to make up for limitations by using a different shell.