std: Add an I/O reader method to fill a buffer

I've found a common use case being to fill a slice (not an owned vector)
completely with bytes. It's posible for short reads to happen, and if you're
trying to get an exact number of bytes then this helper will be useful.
rand: Use fill() instead of read()

It's possible for a reader to have a short read, and there's no reason the task
should fail in this scenario. By using fill(), this has a stronger guarantee
that the buffer will get filled with data.
auto merge of #13049 : alexcrichton/rust/io-fill, r=huonw

This method can be used to fill a byte slice of data entirely, and it's considered an error if any error happens before its entirely filled.