Propose additions to std::prelude

[scottmcm]

Related, and one possible way to do the change:

Random Idea: A new prelude for Rust 2018?

Apparently adding TryFrom to the prelude broke some things, which made me think: Should we have a new prelude version for Rust 2018? Good job whomever made the current prelude prelude::v1; we could do something like prelude::edition2018. Things I don’t know yet: Is this compatible with what we’re allowed to do in the edition? Would writing a lint to warn about future breakage be feasible for this? If we did do this, what should get added to the prelude? Is this even important enough to bothe…

To copy over some things from there (without taking a position either way on them):

• Debug & Display
• HashMap & HashSet
• FromIterator
• Read & Write