[WIP] Implement RFC 41: `lib.fixed` #3739

Job	Run time
smoke (amaranth-lang/amaranth-stdio)	54s
smoke (amaranth-lang/amaranth-boards)	34s
document	56s
smoke (amaranth-lang/amaranth-soc)	58s
check-links	50s
test (pypy-3.9)	5m 9s
test (3.9)	3m 44s
test (pypy-3.10)	4m 57s
test (3.11)	2m 59s
test (3.12)	5m 33s
test (3.13)	3m 25s
test (3.14-dev)	2m 26s
test (pypy-3.11)	4m 46s
test (3.10)	2m 47s
publish-docs	9s
publish-docs-dev	0s
required	2s
publish-package	17s
publish-schemas	6s
publish-release	0s
	40m 32s

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