A simplified version of reduce.
Inject takes a collection and a block. It reduces the collection to a single result thanks to the block.
The block is run for each element in the collection, it's passed
res and elt. res is the result, the accumulator, elt
is the current element in the collection.
The block must return the result for the next iteration.
inject [ '0', 1, 'b', 3 ]
res + elt
# --> "01b3"
An initial value is accepted (generally after the collection)
inject [ 0, 1, 2, 3, 4 ] 10
res + elt
# --> 20
Iterating blocks are given 3 to 4 local variables.
A block iterating over an array will receive elt (the current element
of the iteration), idx (the zero-based index of the current element),
and len (the length of the array).
A block iterating over an object will receive key (the current string
key), val (the current value), idx (the zero-based index of the
current key/val), and len (the length of the object).