Create new non-root nodes with new Array(nodeSize)

...instead of []. In the append benchmark, this yields 30% speedup
with the bit trie and 40% speedup with the vector trie. Because of
this, the vector trie now leapfrogs the bit trie to be faster on the
append benchmark.
Remove naive implementation from benchmark runner
Remove unused pop argument
Add lookup benchmarks & make others consistent

This introduces two benchmarks for lookup, one sequential, one in
random order. Also makes all benchmarks consistent in repeating the
operation under test 1000 times.
Make bit trie the default implementation

It's 80% faster on gets and comparable on sets/pushes.
Add a bunch of slice tests

I may have gotten carried away here! I'm planning to introduce a
wrapper you get when you call vector.slice(), and wanted to try
catching all possible things that could go wrong.