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.
doc: Update the tutorial about bounds for traits
auto merge of #13050 : alexcrichton/rust/no-send-default, r=huonw

See #10296 for the rationale, and commits for the implementation.