Fix issue #271

Buttons should now behave correctly when they are stacked vertically
Fix #557

Code seemed to be from before the switch to the new string
representation.
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?
Resolve issue with getting negative indices

Negative indices produced <internal value> instead of an error, and the
safer versions of get weren’t safe for negative values.
Stopped event propagation in clickables.
Merge pull request #559 from pseudonom/master

Stopped event propagation in clickables.
Remove Array for now, migrate to vector branch

I got around to properly doing my research on various implementations
for immutable vectors. It is not clear to me that the RRB-Trees are
going to be the best trade-off for the typical uses seen in Elm, so I
want to try the Clojure approach as well and race them.