Add hashes to unit-ids created by hadrian

This commit adds support for computing an inputs hash for packages
compiled by hadrian. The result is that ABI incompatible packages should
be given different hashes and therefore be distinct in a cabal store.

Hashing is enabled by the `--flag`, and is off by default as the hash
contains a hash of the source files. We enable it when we produce
release builds so that the artifacts we distribute have the right unit
ids.