AMP refactor

mixed-type args! wahoo and a slightly simpler protocol. This should be interesting too because the quick C AMP implementation I wrote easily does 8+ million ops/s so we should be able to have a cool C client as well. benchmarks seem more or less the same, but there's room to optimize in node-amp's streaming parser, I re-encode to a single `Buffer` which is not great, but I wanted to keep the stream out of `objectMode`, but we could special-case here later