Improve prime number functions for Ruby

- Rewrite 3, 7 and 10 with Ruby
- Rewrite prime number functions using lazy enumerable and keyword arguments
Rename prime.rb to primes.rb
Add 35 with Ruby

- Add 35 with Ruby
- Extract digits manipulation functions for Ruby
Add 50 with Ruby

- Add 50 with Ruby
- Add map_reduce and flat_map_reduce functions for enumerable in Ruby
Add 37 with Ruby
Add 31 with Ruby

- Add 31 with Ruby
- Rewrite 31 with CoffeeScript to use the tabularization method
Improve sequences functions with Ruby

- Rename lazy.rb to sequences.rb
- Rename map_reduce and variants to map_inject
- Add Object#unfold
- Add Integer#and_beyond
Add 10 with Scheme
Improve 18 with Scheme

- Add racket shebang
- Replace some parenthesis with square brackets
Rename prime.coffee to primes.coffee
Rewrite 1 to 9 with Ruby and CoffeeScript