Rename std::borrow to std::reference.

Fixes #11814
Remove double-use of logging.
Remove seldom-used std::reference functions.
Implement default type parameters in generics.
auto merge of #11217 : eddyb/rust/generic-default-type-params, r=cmr
auto merge of #11895 : xales/rust/libstd, r=alexcrichton

Fixes #11814
move concurrent stuff from libextra to libsync