make performance and PEX file size regression test scripts

[cosmicexplorer]

In https://github.com/pantsbuild/pex/pull/1032#issuecomment-695337693, when we thought we would have to vendor the typing module from python 2, the concern was raised that this could affect runtime performance when importing the code in .bootstrap/ (more important), as well as overall PEX file size (somewhat less concerning). It would be neat to have canonical scripts to do this testing for further changes. This would additionally ease the use of git bisect -s in the future.

It's quite likely there will be more than one relevant performance and/or file size scenario we'd be interested in, for different types of changes made to Pex. We'd also likely want to make use of the interpreter selection capabilities of pyenv which are selected between and exposed from tox.ini, in order to easily run this testing over all supported interpreter versions.

It would probably be best to create these starting when any real need arises, and adapt them after that.