Initial partial SPI support

[marnix]

Here is an initial SPI support PR; still draft for now, see below.

It adds an API much like in Discord #microzig message https://discord.com/channels/824493524413710336/824629907089784832/947090519484411945.

And an implementation for that API for the STM32F303.

Here is some code that I've got working on my STM32F3DISCOVERY revision E.02 board, which has an I3G4250D gyroscope connected to the STM32's SPI1 with CS on pin PE3:

        const spi1 = try microzig.Spi(1).init(.{});
        var gyro = spi1.device(microzig.chip.parsePin("PE3"), .{});

        var gyro_id: u8 = undefined;
        {
            var transfer = try gyro.beginTransfer();
            defer transfer.end();
            // read no-autoincrement WHO_AM_I = 0x0F
            try transfer.writer().writeByte(0x80 + 0x0F);
            gyro_id = try transfer.reader().readByte(); // returns 0xD3
        }

What really must go in here before merging:

• Do a clean split between bus configuration (like setting SPIx_CR SPE to enable an STM32 SPI bus) and device configuration (like clock polarity+phase).
• Actually use the PE3 pin parameter, it is currently hardcoded in the STM32F303 implementation.
• Code clean-up, and more/better doc comments.

What should be in here before merging:

• spi.zig support for 'registers' like the I2C implementation has.
• spi.zig support for transceive...() functions.

Update Dec 31, 2022, 15:03 UTC: All of the above was completed, I consider this PR ready for review, removing draft status.

What I would like to leave for later:

• Make clock speed configurable (instead of choosing slowest available).
• Support more SPI buses.
• Allow support for SPI devices with different clock phase and/or polarity.
• Read various status bits and return new ReadError or WriteError values as appropriate.
• A replacement for the hack to wait long enough for the SPI device to detect 'end of transfer'.
• Support data frame sizes other than 8 bits, and related MSB/LSB configuration.
• Support DMA-based reads/writes.
• Support interrupt-based operation.
• Support SPI slave mode.