[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/drv.uefi_fb/module.toml

@@ -0,0 +1,10 @@
+name = "drv.uefi_fb"
+targets = ["user"]
+deps = ["libc"]
+sources = [
+	"font.cpp",
+	"main.cpp",
+	"screen.cpp",
+	"scrollback.cpp",
+]
+