gateware.registers: convert to native amaranth

applet.audio.yamaha_opx: convert to native amaranth Fixes #150.

glasgow.applet: squash ResourceWarning

applet: avoid UnusedElaboratable warning spam in hardware testcases The hardware tests using `applet_hardware_test` work by constructing the whole applet (including the gateware), then discarding the gateware and connecting the applet software to a mock interface. This results in a lot of `UnusedElaboratable` warnings in CI logs, making them effectively unusable. This patch performs a dummy elaboration of the hierarchy to mark all involved elaboratables as used.

applet.program.ice40_sram: add missing Elaboratable inheritance

gateware.uart: remove uses of nbits

applet.interface.ps2_host: fix reset handling

gateware.i2c: convert to native amaranth

gateware.clockgen: convert to native amaranth

applet.*: fix up stray deprecated FIFO signal names