Make build configurations typesafe and factor into separate module
Add type signatures
Make build architecture typesafe
Make build platform a newtype wrapper and provide platform specific constructors
Return list of build products from library rules

For more seamless interaction with `want` and `need`.
Add missing module
Factor pkgConfig into separate module
Factor lens utilities into separate module
Fix "silent" syntax anomality
Add preliminary support for building universal binaries on OSX

Closes #11
Library rules return single path again for more type safety