Auto merge of #5032 - matklad:lazycell, r=alexcrichton

Auto merge of #5032 - matklad:lazycell, r=alexcrichton

Switch to lazycell from crate.io

This switches from a home-grown implementation of `lazycell` to the one from crates.io.

There are no particularly large improvements here, but our own lazy cell is definitely unsafe in theory, because of potential reentrancy in `get_or_try_init`, and the one from crates.io does not have at least this hole :-)

Note that `rustc` already has `lazycell` in its Cargo.lock (because of clippy I guess?), albeit with a lower version, 0.5.