Split asset symbols

[mischnic]

Originally implemented in May 2023

Split assets per-symbol to ideally achieve better codesplitting (especially with huge/prebundled input).

reexporting facade --reexports--> asset for export 1
                   --reexports--> asset for export 2
                   --reexports--> asset for export 3

This is a very old branch so I'm opening this so it doesn't get lost in the depths of Git if somebody wants to test it.
My real-world tests didn't show significant change, I don't know why that was the case.