Work on docs and simplify/normalize API

Did some renaming:
  fill -> repeat
  concat -> append
  getSafe -> getWithDefault

Added initialize function.

Removed some non-core functions that were implemented with lists (zip,
zipWith). Don’t want to provide functions with a slow implementation
and give people a false expectation of speed. These can definitely be
added back at some point because they are pretty handy.

Also got rid of indices because I think it’s easy enough to do [ 0 ..
A.length array - 1 ] and it’s not so common that this would be
desirable in the first place. Also, how do you spell that damn word?