Perf: join is 2x faster 🔥

[maartenbreddels]

I've benchmarked the following:

import vaex
import numpy as np
df = vaex.open('s3://vaex/taxi/yellow_taxi_2009_2015_f32.hdf5?anon=true')[:400_000_000]
dfg = df.groupby('passenger_count', agg=vaex.agg.mean('fare_amount'))
for i in range(20):
    df.join(dfg, 'passenger_count')

And we went form from 113 seconds to 52 (so >x2 speedup), and using 1/9th of the memory. We were always using int64, and always a byte mask, we now let the data decide.
🔥