Create a zero.rs macro. Use it in std.

[brson]

Convert zero.rs into a macro and use it to build std. Add a test case to run-pass that verifies others can use it to bootstrap their freestanding projects.

[brson]

Make sure there's a sensible upgrade path.

[catamorphism]

(bug triage) Not a milestone blocker IMO.

[brson]

zero.rs is no longer really relevant, though I still think it's a good long term goal to build std on top of a more general lower-level substrate. Currently that's looking more like @thestinger's rust-core project.

[thestinger]

Closing in favour of #10043. I'm not entirely sure what the best path here will be, but it's not currently possible due to lacking static linking of crates. It's also not going to be as simple as one configuration flag, because we'll probably need to split the freestanding and C runtime use cases.