Allow constant expressions to refer to other constants

[marijnh]

const tag_a: uint = 12u;
const first_free_tag: uint = tag_a + 1u;

This'll require some resolution pass that complains about dependencies. It'll make implementing #1193 viable.

[marijnh]

Actually, if we stick to our 'no init-time computing' policy, it won't be possible to support this for crate-external consts. Those aren't available at compile time, so if you say const x: int = std::int::max_value - 1;, the const will have to be filled in during initialization.

(Or we could just grab the current value from the .so, but that seems fragile.)

[graydon]

I think the crate-external part can probably block on figuring out how we're going to do cross-crate inlining. It's the same issue. If you export a constant as "inline" you should be able to use it in another crate as part of another const; the value gets copied across at compile time. Otherwise not.

(aside: it is sometimes held among language designers that inline-ness should be declared at the call site not the definition-side; if we go down that road then "const x = external::foo" could well count as an inline-requiring-use. or require some kind of expression-form declaration like "const x = inline(external::foo) + 1")

[catamorphism]

Now that CCI is implemented, it should be easy to finish this?