[project] Clean up src/ tree

A long overdue cleanup of the src/ tree.

- Moved src/drivers to src/user because it contains more than drivers
- Removed src/drivers/ahci because it's unused - will restore it when I
  make a real AHCI driver
- Removed unused src/tools
- Moved kernel.ld (the only used file under src/arch) to src/kernel for
  now, if/when there's a multi-platform effort that should be figured
  out as part of it
- Removed the rest of the unused src/arch
- Renamed 'fb' to 'drv.uefi_fb' and 'nulldrv' to 'testapp'

src/user/testapp/io.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <stdint.h>
+
+extern "C" {
+
+/// Read a byte from an IO port.
+/// \arg port  The address of the IO port
+/// \returns   One byte read from the port
+uint8_t inb(uint16_t port);
+
+/// Write a byte to an IO port.
+/// \arg port  The addres of the IO port
+/// \arg val   The byte to write
+void outb(uint16_t port, uint8_t val);
+
+/// Pause briefly by doing IO to port 0x80
+/// \arg times  Number of times to delay by writing
+void io_wait(unsigned times = 1);
+
+}
+
+constexpr uint16_t COM1 = 0x03f8;
+constexpr uint16_t COM2 = 0x02f8;