class Readable

Reads up to 'count' bytes at the seek offset.

The seek offset is moved forward by the number of bytes read.

## Invariants

* The returned `data.length` will never be greater than `count`.

* If `data.length` is less than `count`, it means that the seek offset

has reached the end of file as part of this operation.

* If `data.length` is zero while `count` is not, it means that the

seek offset is already at or beyond the end of file, and no data could

be read.

* If `count` is zero, the server should perform all the checks ensuring

read access without actually read anything, and return an empty

`data` vector.

This method requires the [`Rights.READ_BYTES`] right.

Returns `ZX_ERR_OUT_OF_RANGE` if `count` is greater than `MAX_TRANSFER_SIZE`.