Support an unzip toggle for PEXes.

[jsirois]

This adds a PexInfo.unzip toggle and --unzip --no-unzip in the
CLI to mark a PEX file that should be unzipped before execution.
Combined with a stable PEX_ROOT cache, this can offer improved startup
latency in both cold and hot cases.

Work towards #930.

[jsirois]

Using @illicitonion's benchmark in #930

Baseline old pex:

$ multitime -n20 ./trivial.pex >/dev/null
===> multitime results
1: ./trivial-slow.pex
            Mean        Std.Dev.    Min         Median      Max
real        0.422       0.007       0.414       0.419       0.440       
user        0.383       0.011       0.365       0.383       0.406       
sys         0.035       0.010       0.017       0.036       0.053

For the warm cache case goes down to:

$ multitime -n20 ./trivial.pex >/dev/null
===> multitime results
1: ./trivial.pex
            Mean        Std.Dev.    Min         Median      Max
real        0.263       0.004       0.255       0.263       0.272       
user        0.230       0.009       0.201       0.230       0.244       
sys         0.032       0.010       0.017       0.030       0.060       

But even the cold case becomes faster:

$ PEX_ROOT=/tmp/cache multitime -n20 -r 'rm -rf /tmp/cache' ./trivial.pex >/dev/null
===> multitime results
1: -r "rm -rf /tmp/cache" ./trivial.pex
            Mean        Std.Dev.    Min         Median      Max
real        0.367       0.003       0.359       0.367       0.373       
user        0.328       0.009       0.305       0.328       0.345       
sys         0.038       0.008       0.023       0.036       0.053      

[Eric-Arellano]

Great results!

[illicitonion]

Very nice! Thanks!!