Backporting futures

[gnunicorn]

from @chavic 's working branch into latest main:

• reactives the future tests
• copies over the futures implementation code
• for that already takes over the new uniffi_function generation code (but not yet the singleton)
• expands it over object methods
• fixes the hello-world test failure problem
• cleans up old, not necessary code and prints

Best viewed one commit at a time as these explain in the name what is even happening.

[gtalha07]

Looks good! Although you probably wanna look at some test fails (but irrelevant to this PR)