Auto merge of #21680 - japaric:slice, r=alexcrichton
Auto merge of #21680 - japaric:slice, r=alexcrichton

Replaces `slice_*` method calls with slicing syntax, and removes `as_slice()` calls that are redundant due to `Deref`.

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