applet.memory._25x_passthrough: new applet

This has now been implement based on two datasheets:
- Winbond W25Q80DVUXIE
- Macronix MX25L6436F

These chips implement incompatible ways to enter XIP mode.
Not all the commands have been tested, only the following:
- Non-quad/non-dual commands that go directly to "Wait-Cs-Deassert", and never
  change the IO directions
- Command 0xEB that reads data over 4 i/o pins, and this has only been tested
  in XIP mode.
- Going directly back into XIP mode after CS deassert-assert.
- I can get an RP2040 to run through the passthrough, by using the
  `RX_SAMPLE_DLY` functionality at up to 77.5MHz SPI clock. Higher speeds are
  not possible on the RP2040, because RX_SAMPLE_DLY can only be set up to `4`,
  but the applet may be capable of more with other systems.

Author: purdeaandrei