Two-dimensional matrix backed by Array from the Elm core, the fast immutable array implementation.
The main motivation for this is the indexedMap function that is useful in games where you
want to act depending on position as well as content. E.g., the game of life where the cell
is depending on its neighbours.
Speaking of neighbours, that is also a function in this package.
API design follows the Elm core Array module.
Naming is now rows and columns, as it is more fit to the purpose of the library.
New function foldl for folding left over each row and then merge.