Add array @reduce, @filter, @any, @all, & @indices_of

[NotsoanoNimus]

Add another wave of new core array operations to the standard library.

In brief:

1. @reduce - apply a function iteratively to an array and store the result into an accumulator.
   a. @sum - terse way to sum up all array elements with @reduce.
   b. @product - terse way to multiply all array elements with @reduce.
2. @filter - creates a shallow copy of an array, containing only copies of elements which match a given predicate function.
3. @any - return true when any element in the array matches a given predicate function.
4. @all - return true when all elements in the array match a given predicate function.
5. @indices_of - underlies the @filter macro; returns the indexes of the array which match the given predicate function.

Please see the test/unit/stdlib/core/array.c3 file for a detailed look at each function's unit testing. Also, these functions (like @zip ones) are compatible with List types and other iterables.

[lerno]

Thank you!