Add a suggestion to use a `let` binding on some borrowck errors.
auto merge of #14350 : zwarich/rust/let-suggestion, r=pcwalton
libstd: Remove `~str` from all `libstd` modules except `fmt` and `str`.
libtime: Remove the `tz_zone` field from times.

It depends on `~str`.
libstd: Remove all uses of `~str` from `libstd`
libcore: Remove all uses of `~str` from `libcore`.

[breaking-change]