Do not use `Optional` class name explicitly

This commit fixes mutant failure:

    $ mutant --fail-fast --include lib --require monads --use rspec
    'Monads::Optional'
    #<Method: Monads::Optional.[]> does not have valid source location
    unable to emit matcher
    #<Method: Monads::Optional.members> does not have valid source location
    unable to emit matcher
    #<Method: Monads::Optional.new> does not have valid source location
    unable to emit matcher
    #<UnboundMethod: Monads::Optional#value> does not have valid source
    location unable to emit matcher
    #<UnboundMethod: Monads::Optional#value=> does not have valid source
    location unable to emit matcher
    Mutant configuration:
    Matcher:            #<Mutant::Matcher::Config
    match_expressions=[#<Mutant::Expression::Namespace::Exact
    syntax="Monads::Optional">] subject_ignores=[] subject_selects=[]>
    Integration:        rspec
    Expect Coverage:    100.00%
    Available Subjects: 1401
    Subjects:           2
    Monads::Optional.from_value:/home/peter/devel/monads/lib/monads/optional.rb:17
    mutations: 13
    - rspec:Monads::Optional.from_value
    .........F
    (09/13)  69% - killtime: 0.04s runtime: 0.65s overhead: 0.61s
    Monads::Optional.from_value:/home/peter/devel/monads/lib/monads/optional.rb:17
    - rspec:Monads::Optional.from_value
    evil:Monads::Optional.from_value:/home/peter/devel/monads/lib/monads/optional.rb:17:c1404
    @@ -1,4 +1,4 @@
     def self.from_value(value)
    -  Optional.new(value)
    +  self.new(value)
     end
    -----------------------
    Subjects:  2
    Mutations: 13
    Kills:     9
    Alive:     4
    Runtime:   0.66s
    Killtime:  0.04s
    Overhead:  1556.76%
    Coverage:  69.23%
    Expected:  100.00%