Perform proper refcounting on imported crate-cache entries.

[graydon]

The crate-cache entries (and thus 'use'd crates) currently possess the ability to be flushed and reloaded, but the import calls fail to perform the refcounts on either side of cross-crate calls, and there isno exposed way to ask the runtime to request a flush/reload.

This will require some unwinding work as well, to drop refcounts when unwinding across a cross-crate call. And closure work, for captured imports. And other stuff.

[graydon]

WONTFIX (linkage scheme will change; unloading will no longer be possible)