memorymap: Convert to use memoryview internally.

[jepler]

Importantly, a memoryview can be .cast(), and then accesses to it for 4 byte values don't require allocations or to/from_bytes. (as long as the register values actually fit in CircuitPython "small integers")

This does remove the existing protections enforcing aligned accesses to IO blocks on raspberrypi. However, the behavior in these cases is actually well defined: an 8 or 16 bit write access is replicated across all 32 bits of the register. See RP2040 datasheet 2.1.4. Narrow IO Register Writes and RP2350 datasheet 2.1.5. Narrow IO register writes.

As discussed on Discord, this speeds up the keyboard scanner of my Unicomp Mini M firmware by nearly 10x compared to using DigitalInOut, mostly because 12 pins can be read by a single operation, allowing it to scan the full keyboard in under 1ms when overclocked. (https://adafruit-playground.com/u/jepler/pages/unicomp-mini-m-with-circuitpython currently details the original lower performance keyboard, which took about 6ms to scan the full keyboard when overclocked)

Currently tested only on rp2040.

[jepler]

Here's the code I use in my program to read & update GPIO pins as a group:

SIO_BASE = const(0xd0000000)
SIO_LEN = const(0x1000)
GPIO_IN = const(4//4)
GPIO_OUT_XOR = const(0x1c//4)

sio = memorymap.AddressRange(start=SIO_BASE, length=SIO_LEN).cast('L')

def gpio_in():
    """Fetch the status of all GPIO inputs"""
    return sio[GPIO_IN]

def gpio_out_xor(x):
    """Invert the given pins simultaneously"""
    sio[GPIO_OUT_XOR] = x

[jepler]

Space Savings from this PR, raspberry_pi_pico:

• 840b flash
• 256b RAM
  The RAM savings could be had in main just by making the array of regions be "const", so that it resides in flash and is not copied to RAM.