Update to Amaranth 0.5

[miek]

Work-in-progress on updating to Amaranth 0.5:

• [x] Replace uses of amaranth.hdl.Memory with amaranth.lib.memory.Memory
• [x] Remove uses of Signal.width
• [x] Replace uses of reset= with init=
• [ ] Remove uses of Pin
• [ ] Update tests to remove uses of add_sync_process
• [ ] ...