Warnings in compiler

[kach]

We need infrastructure to emit compiler warnings, e.g. for version mismatch (#245).

Seems like the eresult type could be modified such that a Success includes both the result and a list of warnings. Then by modifying elift appropriately, it shouldn't be hard to integrate this change into the codebase.

[jeromesimeon]

Infrastructure for warnings added in #625 and cleaned up in #630