add a simple benchmark (reading a vector tile)
replace switch with ifs (they're faster in JS)

improves vector tile reading by ~8%
cache buffer variable for a small boost (3%)
60% faster readPacked
faster packed writes as well
fix bug with realloc on empty buffer
25% faster writeVarint

+val is the same as Number(val) but slower
Merge pull request #6 from mapbox/bench

Simple benchmarks and performance improvements