Impl Bake for BTreeSet

[adamchalmers]

This is my attempt to start #4266.

I am confused why the unit test I added is failing -- I might be missing something about how quote! works.

thread 'alloc::hash_set' panicked at utils/databake/src/alloc.rs:75:5:
assertion `left == right` failed
  left: "alloc :: collections :: BTreeSet :: from ([1u8 , 2u8 ,])"
 right: "alloc :: collections :: BTreeSet :: from ([1u8 , 2u8])"

IMO those two should be the same, but I'm not sure why the left has a trailing comma. I'd appreciate help from anyone who understands what's going on there!

[CLAassistant]

All committers have signed the CLA.

[Manishearth]

Pushed the test fix for you. The token tree testing stuff is a bit tricky around trailing commas.

[adamchalmers]

Thanks very much Manish!