auto merge of #13049 : alexcrichton/rust/io-fill, r=huonw
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.

waynenilsen/rand,retep998/rand,bluss/rand,huonw/rand,ebfull/rand,GrahamDennis/rand,arthurprs/rand,shepmaster/rand,achanda/rand,bhickey/rand