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.

dev/input.c

@@ -60,7 +60,8 @@ kern_status_t input_device_report_event(struct input_device *dev, const struct i
 	return r == sizeof *ev ? KERN_OK : KERN_WOULD_BLOCK;
 }
 
-kern_status_t input_device_read(struct device *dev, void *buf, size_t size, size_t *bytes_read, socks_flags_t flags)
+kern_status_t input_device_read(struct device *dev, void *buf, size_t offset,
+                                size_t size, size_t *bytes_read, socks_flags_t flags)
 {
 	if (dev->dev_type != DEV_TYPE_INPUT || (size % sizeof (struct input_event)) != 0) {
 		return KERN_INVALID_ARGUMENT;