A step towards amaranth `v0.5.0` #2419

Job	Run time
Synthesize full core	29s
Build regression tests (riscv-tests)	45s
Build regression tests (riscv-arch-test)	2m 57s
Check code formatting and typing	35s
Run unit tests	7m 47s
Run regression tests (riscv-tests)	2m 27s
Run regression tests (riscv-arch-test)	10m 47s
	25m 47s

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