Patch-level release 1.16.4

This release:

• adds pio::sm-clock! on RP2040 and RP2350 platforms to set the clock divider of a PIO state machine to approximate a given Hz based on the current value of sysclk.
• modifies the CYW43439 SPI driver at extra/rp_common/cyw43/cyw43_spi.fs to use pio::sm-clock!.
• modifies the WS2812 driver at extra/rp_common/neopixel.fs to use pio::sm-clock! to set the PIO clock divider, in the process fixing issues with it on the RP2350.