Migrate merge() from stdlib and make it support iterative creation of hash

I think we should migrate the merge() function from stdlib to puppet and teach it new tricks.

Currently it only merges two hashes - it should instead be allowed to iterate over a hash or array and yield to a lambda that computes entries to merge - either by returning a hash, or an array that can be converted to a Hash. Returning undef adds nothing.

This is of value since the only option in puppet to iteratively building a hash is via the reduce function (or by first creating an array - with similar issues) and it does not perform well if used to build a large hash (it will create a new copy of the hash it is building on each iteration).

The lambda should be called with the hash built to this point (to allow reading it).

$input = [a, b, c, d, c, b, b, a]

$input.merge |$hsh, $val| { { $val => $hsh[$val].lest || { 0 }  + 1} }

Would build a hash with a count per occurence of string in an array. The example would produce:

{ a => 2, b => 3, c=> 2, d => 1 }