Change static.rust-lang.org to doc.rust-lang.org

The new documentation site has shorter urls, gzip'd content, and index.html
redirecting functionality.
doc: Fix some broken links
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]
doc: Touch up the unsafe guide

* Change ~ references to Box
* Rewrite examples so they can be compiled an run
* Mention libcore
* Update wording about compiler-required functions
auto merge of #14310 : pcwalton/rust/detildestr-alllibs, r=brson

r? @brson
auto merge of #14348 : alexcrichton/rust/doc.rust-lang.org, r=huonw