Do not use `Many` class name explicitly

This commit fixes mutat failure:

    $ mutant --fail-fast --include lib --require monads --use rspec
    'Monads::Many'
    #<Method: Monads::Many.[]> does not have valid source location unable to
    emit matcher
    #<Method: Monads::Many.members> does not have valid source location
    unable to emit matcher
    #<Method: Monads::Many.new> does not have valid source location unable
    to emit matcher
    #<UnboundMethod: Monads::Many#values> does not have valid source
    location unable to emit matcher
    #<UnboundMethod: Monads::Many#values=> does not have valid source
    location unable to emit matcher
    Mutant configuration:
    Matcher:            #<Mutant::Matcher::Config
    match_expressions=[#<Mutant::Expression::Namespace::Exact
    syntax="Monads::Many">] subject_ignores=[] subject_selects=[]>
    Integration:        rspec
    Expect Coverage:    100.00%
    Available Subjects: 1401
    Subjects:           2
    Monads::Many.from_value:/home/peter/devel/monads/lib/monads/many.rb:13
    mutations: 17
    - rspec:Monads::Many.from_value
    .........F
    (09/17)  52% - killtime: 0.04s runtime: 0.70s overhead: 0.66s
    Monads::Many.from_value:/home/peter/devel/monads/lib/monads/many.rb:13
    - rspec:Monads::Many.from_value
    evil:Monads::Many.from_value:/home/peter/devel/monads/lib/monads/many.rb:13:f896c
    @@ -1,4 +1,4 @@
     def self.from_value(value)
    -  Many.new([value])
    +  self.new([value])
     end
    -----------------------
    Subjects:  2
    Mutations: 17
    Kills:     9
    Alive:     8
    Runtime:   0.70s
    Killtime:  0.04s
    Overhead:  1780.39%
    Coverage:  52.94%
    Expected:  100.00%