dev: implement reading from block devices

reading from block devices is done using the block cache (bcache).
This cache stores sectors from a block device in pages of memory
marked as 'cached', which will allow them to be reclaimed when
memory pressure is high (TODO).

while block device drivers implement callbacks allowing reading/writing
at block-granularity, the device subsystem uses the block cache to
implement reading/writing at byte-granularity in a driver-agnostic way.

block drivers can disable the block cache for their devices, but this
will require that any clients communicate with the devices at
block-granularity.

also added an offset parameter to device and object read/write functions/callbacks.

kernel/tty/device.c

@@ -33,7 +33,7 @@ static void tty_console_write(struct console *con, const char *s, unsigned int l
 {
 	if (foreground) {
 		size_t nr_written;
-		tty_write(foreground, s, len, &nr_written, 0);
+		tty_write(foreground, s, 0, len, &nr_written, 0);
 	}
 }