libstd: Document the following modules:

* native::io
* std::char
* std::fmt
* std::fmt::parse
* std::io
* std::io::extensions
* std::io::net::ip
* std::io::net::udp
* std::io::net::unix
* std::io::pipe
* std::num
* std::num::f32
* std::num::f64
* std::num::strconv
* std::os
std: Touch various I/O documentation blocks

These are mostly touchups from the previous commit.
auto merge of #13135 : alexcrichton/rust/dox, r=alexcrichton

I touched up the documentation from @pcwalton found in #12952.
rustc: Stop adding the Send bound implicitly

This commit removes implicitly adding the Send bound to ~Trait objects and
procedure types. It will now be manually required to specify that a procedure
or trait must be send-able.

Closes #10296
Fix fallout of removing default bounds

This is all purely fallout of getting the previous commit to compile.