A step towards amaranth `v0.5.0` (#577) #2420

Job	Run time
Build regression tests (riscv-tests)	44s
Synthesize full core	32s
Build regression tests (riscv-arch-test)	3m 40s
Check code formatting and typing	38s
Run unit tests	8m 5s
Run regression tests (riscv-tests)	2m 36s
Run regression tests (riscv-arch-test)	10m 22s
	26m 37s

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