RSpec beta2
Memoize sources_for call in dispatch.

30% speedup:

    # Before
    > ruby benchmark/method_dispatch.rb
    #dispatch         0.180000   0.000000   0.180000 (  0.183436)
    #eager_dispatch   0.180000   0.000000   0.180000 (  0.182031)

    # After
    > ruby benchmark/method_dispatch.rb
    #dispatch         0.140000   0.000000   0.140000 (  0.139582)
    #eager_dispatch   0.140000   0.000000   0.140000 (  0.139984)

w/ @strathmeyer