Replace Buffer with Uint8Array

[valadaptive]

I still need to do some cleanups and replace uses of Buffer.compare, but I'm putting this PR here so you can benchmark it. Before I can complete this, #428 (minus the version bump) should be merged in, as the services code uses Buffer everywhere and expects fixed/bytes types to decode to Buffers.

I've made some tweaks to the benchmarking setup, so comparing this directly to the master branch won't work:

• I've added an ArrayFloat benchmark to go along with ArrayDouble.
• I use the --expose-gc flag when benchmarking in order to manually trigger garbage collection between benches, hopefully making results more consistent.
• I've changed the length distribution of strings to be exponentially weighted, so shorter strings are still likely but longer strings will now be occasionally generated. The previous code was only benchmarking the manual path of Tap#writeString, since it only generated strings up to a length of 32.

I've cherry-picked those benchmarking changes into the bench-tweaks branch, which you can use to compare benchmarks.

[mtth]

Thanks @valadaptive! I'll try to find time to merge #428.

[mtth]

FYI @valadaptive - #428 is in.

[valadaptive]

Working on removing Buffer usage from types.js now. I noticed the isJsonBuffer function, which seems to check if a given object is the JSON representation of a Buffer. Under what circumstances are Avro types directly serialized to JSON and/or parsed back directly? I can't easily polyfill Uint8Array to stringify to a regular array, so I'll probably need to insert a fixup step when stringifying/parsing.