Naming of use_std feature

[dtolnay]

The C-FEATURE guideline discourages calling a Cargo feature "use_std". I would recommend "std" instead.

[BurntSushi]

Funnily enough, I was using std, but switched to use_std just before the release because it's what libc uses.

I think this can be done in a backwards compatible way, but we'd need to keep use_std around. Not sure whether I prefer just use_std or having both...

[ethanpailes]

Personally I would rather have just one way to do it. Small api surface and all that.