MPL issues

See this for analysis of a problem with license = "MPL-2.0":

!1160 (fb594031, comment 2899771)

We should:

• figure out what is a good thing that the Rust community should do
• contact generational-arena upstream to ask them to do this (even if we don't end up using that crate)
• contact option-ext to ask them to do this (since @nickm says we're going to have that in our deps soon)
• Probably, write some blog post for TWIR.