Support cross-language linking with C

See also #15 , but less clear path forward, as Haskell doesn't need an explicit SWIGging phase, can just live with `foreign import`s. See https://groups.google.com/forum/#!topic/kythe-dev/Fb7HmZffRtw for a related discussion.

The core of the matter is that when emitting references to the C code, we need to use the VName emitted by the C indexer, but that is unpredictable (we need side-channel info from the C indexer somehow).

The matter is further complicated by `hsc2hs`, `inline-c` and maybe other glue libs. A proper research is needed. Maybe we don't need to support all the edge cases, or these cases are actually separate instances to solve on their own.