Let hazel_library cover non-library targets

[judah]

We should make hazel_library more generic so it can support targets besides just libraries.

Suggestion: make a generic hazel macro:

def hazel(package, target=None):
  if not target:
    target = package
  else if target == package: # binary
    target = package + "_bin"
  ...

Socialize this design first to see how intuitive it is in practice.