Right now std and getrandom essentially duplicate each other. And Rust already depends on rand (via tempfile) which in v0.7 will use getrandom. So I think it makes sense to use a single implementation, to keep work on correctly retrieving system entropy focused in one place.

Note that right now I do not propose to expose getrandom API as part of std or introduce a lang item, it's a separate discussion, see rust-random/getrandom#21. Also it's probably will be better to wait until rust-random/getrandom#13 will be closed, which should happen relatively soon.

cc @dhardy @tarcieri