fix "warning: deprecated syntax, use `for` keyword now"
Reference: Fix list in Expressions section
auto merge of #18922 : japaric/rust/for, r=jakub-

r? @alexcrichton
auto merge of #18924 : cakebaker/rust/fix_list, r=steveklabnik
Move FromStr to core::str
Remove core::num::strconv
Remove use of deprecated function
Rename IntoStr to IntoString

For consistancy with ToString
Move IntoString to collections::string
Move ToString to collections::string

This also impls `FormatWriter` for `Vec<u8>`