refactor definition of' map', implement new h.o.f.

- rename map' to map
- filter
- positives
- multiples
- foldr
- length_foldr      'length' reimplemented with 'foldr'
- sum_foldr :       'sum' reimplemented with 'foldr'
- product_foldr :   'product' reimplemented with 'foldr'
- cons :            cons operator (':') implemented as a prefix function
add my submission for assignment 1