Investigate removing the units field from Label to conserve memory

[danielsn]

          I actually wonder if we should get rid of the units and take the memory savings that come with that -- I'm not sure any of our profilers ever sets the units.

Originally posted by @ivoanjo in #202 (comment)

[ivoanjo]

To add from the other thread, it looks only PHP is making use of this:

I did a quick check:

• None in Ruby profiler
• Quick check in Python profiler seems none are used
• Don't see it in dotnet
• Also not in the native profiler

...ahaha but last and not least, php does seem to be using it.

But it may be worth discussing and seeing if we can agree to drop this from the API, so we can save some extra memory/overhead.