Use hash for local signatures

As hinted in http://www.kythe.io/docs/schema/writing-an-indexer.html#_cross_references, keeping the signatures in VNames short is crucial for good IO performance.

For non-top/exported entities, now we append disambiguator info like filename, line/col to the signature. Approximately `haskell:term:pkg_name:The.Module:thing:/pkg_name/the/file.hs-123-456`.

We could cut down on this.