Sections Rework

[ZwFink]

This issue is a draft. Pre-requisites: #3459 Requirements:

• Support cross-array sections
• One spanning tree for cross-array sections instead of one tree per array (Eric has asked for this for OA)
• Don't copy msg for every child when doing multicasts
• Don't store the list of elements anywhere
• Don’t collect the whole list of locations?
• No usage pattern restrictions like multicast->reduction->multicast->reduction...
• Lightweight proxy that can be passed around and used to send to the section
• Parameter marshalling
• Tree rebuilding when chares migrate
• Broadcasts/reductions should be node aware
• Distributed creation using split/combine, similar to Charm4Py