Give reduction-type tests in Bitv more natural names
Simplify implementation of Bitv::{all,none} using iter builtins.
auto merge of #13572 : yuriks/rust/bitv-renames, r=alexcrichton

Renamed the methods in Bitv that test all bits to have names that, ehm, "roll off the tongue" a bit more easily. :)

Also added tests for them while at it, since they seemed to be missing.