Do no compile library items that are not used

[mohammadfawaz]

We could do this in two steps:

1. Do not compile whole libraries that are not needed (or transitively needed) by the entry program.
2. Do not compile library items that are not needed (or transitively needed) by the entry program.

(2) will become easier when the collection context work is complete (I think).

[otrho]

(1) is already kinda handled by Forc.toml isn't it? We only compile the libraries in there before providing them to the entry. I guess transitive dependencies of 'whole' libraries still happens unnecessarily, but that sort of analysis might also require the CC.

[mohammadfawaz]

Yeah thinking about "sub-libraries" of the standard library here as an example. If the top level program does not need assert anywhere, even transitively, then don't compile the assert library.