With this pull request, we're introducing a `ZED_MEASUREMENTS` variable
which is used by the `measure` function to print to stderr how long it
took to execute a given closure. This is used right now to measure frame
time when a new frame is requested. This data can be plotted as a
histogram via `script/histogram`.

Here's a workflow I typically use:

- Run `export ZED_MEASUREMENTS=1`
- Checkout a commit that you want to measure the performance of
- Run zed in release mode and use it for 5-10 seconds
- Copy the printed measurements into a file, say `version-a`
- Checkout another commit that you want to measure the performance of
- Run zed in release mode and use it for 5-10 seconds
- Copy the printed measurements into a file, say `version-b`
- Run `script/histogram version-a version-b` (note that you can supply
however many files you want)

Release Notes:

- N/A