Reintroduce futures using Future and Promise #1148

Job	Run time
Tests / cargo-deny	33s
Tests / amd64-linux-gnu	1m 37s
Tests / amd64-linux-musl	2m 35s
Tests / cargo-clippy	38s
Tests / inko-fmt	51s
Tests / cargo-fmt	21s
Tests / vale	10s
Tests / amd64-freebsd-native	3m 59s
Tests / amd64-mac-native	2m 42s
Tests / arm64-mac-native	1m 12s
Standard library documentation	0s
Nightly container	0s
Documentation	0s
	14m 38s

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