Moved all dump()-related functions into a class "serializer". This fix includes a lot of performance improvements yielding a 7% speedup for serialization. Details on the individual steps can be found in the commit messages.

Individual benchmark numbers:

before:

dump jeopardy.json                            5   374555228 ns/op
dump jeopardy.json with indent                5   430953700 ns/op
dump numbers/floats.json                      5   622938509 ns/op
dump numbers/signed_ints.json                20    82177979 ns/op

after:

dump jeopardy.json                            5   335449757 ns/op -11%
dump jeopardy.json with indent                5   375467773 ns/op -13%
dump numbers/floats.json                      5   584611852 ns/op -7%
dump numbers/signed_ints.json                20    68289574 ns/op -17%