gateware.uart,applet.interface.uart: remove ad-hoc inversion. No longer needed as the same result can be achieved with `--pin-rx 0#` and `--pin-tx 1#`.

access.direct: implement selective pin inversion with `--pin-x 0#`. Amaranth supports inversion for any pin and for any I/O core on the I/O buffer level. This mechanism obviates the need for ad-hoc inversion in any applets, but was not exposed until now. Any pin can now be inverted via the command-line interface using one of the following syntaxes: * single pin: `--pin-x 0#` * pin range: `--pins-x 0:8#` (inverts all of them) * pin list: `--pins-x 0,1#,2#,3` (inverts only specified pins) Pull-ups configured for a pin with inversion get converted to pull-downs and vice versa.

access: reimplement `get_deprecated_pad{,s}()` via `get_port()`. There is no loss of functionality and this change allows us to only consider `get_port()` as a consumer of pin argument values.

access: remove `get_inout_fifo()`. There is no advantage to using it over `get_in_fifo()` and `get_out_fifo()`. Only one applet, `selftest`, ever used it at all.

target.hardware: remove workaround for amaranth-lang/amaranth#1402. Not necessary since Amaranth 0.5.1.

applet.interface.i2c_{initiator,target}: modify to use port groups. Also, update the I2C core to handle the same.

software: require amaranth>=0.5.1,<0.6.

applet.interface.swd_openocd: improve formatting of help text.

gateware.ports: add `PortGroup.{__getitem__,__setattr__}`.

platform.generic: delegate `GlasgowPlatformPort.direction` to `.io_port`. This doesn't actually change behavior at the moment, but would be the correct implementation if we ever narrow underlying port directions.