std: add reverse vec iterators, replace vec::each*_reverse.
std: remove {all*,any*,count} in favour of iterators
std: remove fold[lr] in favour of iterators
std: remove foldr and alli methods in vec
std: remove each_char* fns and methods from str, replaced by iterators.
std: replace the str::each* fns/methods with byte iterators
remove unused import warnings
auto merge of #7015 : huonw/rust/each-fn-kill, r=thestinger

Continuation of #6995/#6999.