Make UARTs adhere to the std.Io.Reader interface

[pcolladosoto]

When trying to leverage an UART through an std.Io.Reader interface I was met with compilation errors such as

$ zig build
failed command: /Users/pcollado/.zvm/0.17.0-dev.1158+1d1193aa7/zig build-exe -fno-strip -fsingle-threaded -OReleaseSmall -target thumb-freestanding-eabi -mcpu cortex_m0plus --dep libmodbuz --dep microzig -Mroot=/Users/pcollado/repos/zig/zpi/src/main.zig -OReleaseSmall -target thumb-freestanding-eabi -mcpu cortex_m0plus -Mlibmodbuz=/Users/pcollado/repos/zig/zpi/zig-pkg/modbuz-0.1.0-0Q-9R9XEAgDzydH9_sRFHTaunW6CqI9rsSxKNNETe53K/src/root.zig --dep config --dep drivers --dep cpu --dep chip --dep hal --dep board -Mmicrozig=/Users/pcollado/repos/zig/zpi/../microzig-fork/core/src/microzig.zig -Mconfig=.zig-cache/o/9cc470c215c9d53763a20b54005a414b/options.zig -ODebug --dep link -Mdrivers=/Users/pcollado/repos/zig/zpi/../microzig-fork/drivers/src/root.zig --dep rtt --dep microzig -Mcpu=/Users/pcollado/repos/zig/zpi/../microzig-fork/core/src/cpus/cortex_m.zig --dep microzig -Mchip=.zig-cache/o/a4dc3ed0c6ced3b6b47ac543b2deba81/chips/RP2040.zig --dep bounded-array --dep microzig -Mhal=/Users/pcollado/repos/zig/zpi/../microzig-fork/port/raspberrypi/rp2xxx/src/hal.zig --dep bootloader --dep microzig -Mboard=/Users/pcollado/repos/zig/zpi/../microzig-fork/port/raspberrypi/rp2xxx/src/boards/raspberry_pi_pico.zig -ODebug -Mlink=/Users/pcollado/repos/zig/zpi/../microzig-fork/modules/network/link/link.zig -ODebug -Mrtt=/Users/pcollado/repos/zig/zpi/../microzig-fork/modules/rtt/src/rtt.zig -ODebug -Mbounded-array=/Users/pcollado/repos/zig/zpi/../microzig-fork/modules/bounded-array/src/bounded_array.zig -Mbootloader=.zig-cache/o/01d11e229d79fbdd352e20001304475f/w25q080.bin -ffunction-sections -fdata-sections --gc-sections --cache-dir .zig-cache --global-cache-dir /Users/pcollado/.cache/zig --name zpi -static -fcompiler-rt --script .zig-cache/o/498e1b9ecbd57f16bb8c6a4148dc6d7c/linker.ld --zig-lib-dir /Users/pcollado/.zvm/0.17.0-dev.1158+1d1193aa7/lib/ --listen=-

install
└─ install generated to zpi-dbg.elf
   └─ compile exe zpi-dbg Debug thumb-freestanding-eabi 3 errors
/Users/pcollado/repos/zig/microzig-fork/port/raspberrypi/rp2xxx/src/hal/uart.zig:178:27: error: missing struct field: seek
            .interface = .{
                         ~^
/Users/pcollado/repos/zig/microzig-fork/port/raspberrypi/rp2xxx/src/hal/uart.zig:178:27: note: missing struct field: end
/Users/pcollado/.zvm/0.17.0-dev.1158+1d1193aa7/lib/std/Io/Reader.zig:1:1: note: struct declared here
const Reader = @This();
^~~~~
referenced by:
    init: src/modbus.zig:161:30
    main: src/main.zig:87:16
    6 reference(s) hidden; use '-freference-trace=8' to see all references
/Users/pcollado/repos/zig/microzig-fork/port/raspberrypi/rp2xxx/src/hal/uart.zig:210:28: error: @fieldParentPtr increases pointer alignment
        const r: *Reader = @fieldParentPtr("interface", io_reader);
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/pcollado/repos/zig/microzig-fork/port/raspberrypi/rp2xxx/src/hal/uart.zig:210:28: note: parent pointer type '*hal.uart.UART.Reader' has alignment '2'
/Users/pcollado/repos/zig/microzig-fork/port/raspberrypi/rp2xxx/src/hal/uart.zig:155:24: note: struct field 'interface' limits alignment to '2'
    pub const Reader = struct {
                       ^~~~~~
/Users/pcollado/repos/zig/microzig-fork/port/raspberrypi/rp2xxx/src/hal/uart.zig:210:28: note: use @alignCast to assert pointer alignment
src/modbus.zig:44:30: error: no field or member function named 'take_byte' in '?hal.uart.UART.Reader'
    const count = uart_reader.take_byte() catch |err| {
                  ~~~~~~~~~~~^~~~~~~~~~

$ zig version
0.17.0-dev.1158+1d1193aa7

This PR adds changes so that the UART's HAL adheres to the std.Io.Reader interface whilst also updating the stream function.

The reader implementation works on my side when driving the UART for performing Modbus requests.

If there's anything that should be changed don't hesitate to do so: these are my first steps with Zig!

Thanks a ton for your time.