Implement `case_of`

[rap1ds]

Maybe implements the threequals method in a way which allows it to be used in case expressions pretty neatly: You can match both the type (None or Some) and the value of Some. It almost feels like pattern matching in functional language. However, there's one big shortcoming. Ruby doesn't allow binding the value of Some to a variable, so you must use the get method to do that. The following does not work in Ruby:

case apples
when Some(1)
  puts "one apple"
when Some(x)
  puts "#{x} apples"
when None
  puts "Unknown number of apples"
end

Instead, you have to use get, which is cubersome:

case apples
when Some(1)
  puts "one apple"
when Some
  x = apples.get
  puts "#{x} apples"
when None
  puts "Unknown number of apples"
end

To workaround this limitation, Maybe implements a method case_of, which internally uses case expression:

apples.case_of(Some(1)) { |_| puts "1 apple" }
apples.case_of(Some(Integer)) { |x| puts "#{x} apples" }
apples.case_of(None) { |_| puts "Unknown number of apples" }

- apples.case_of(Some(1)) do
  = "1 apple"

- apples.case_of(Some(Integer)) do |x|
  = puts "#{x} apples"

- apples.case_of(None) do
  = "Unknown number of apples"