Rename std::borrow

[brson]

We don't call these 'borrowed pointers' anymore, but 'references'. Maybe std::ref. Nominating.

[huonw]

ref is a keyword and so doesn't work as a module name.

[glaebhoerl]

reference? :)

[omasanori]

BTW is the word 'region pointers' also obsoleted?

[brson]

@omasanori yes, i think references and lifetimes are preferred to both borrowed pointers and regions.

[omasanori]

@brson Thank you.

[pnkfelix]

Accepted for 1.0, P-backcompat-libs