Support scala 2.13 (to enable full spark 3.2.0 support)

[skestle]

Issues with:

• StructType string changes
• Fragile schema tests because scala's .groupBy function is not deterministic in ordering, and deep schemas don't resolve
  The existing tests on 2.12 should be able to be falsified just be adding a few more foo.* columns in the nested DataFrame

[skestle]

In terms of a schema-equality breaking test - are the following 2 equal?:

• root
  • foo
    • baz
    • bar
• root
  • foo
    • bar
    • baz

I believe they are (given that we make a lookup map for schema equality). I can add the test tomorrow if you like, but I'm not sure I want to do the code fix ;). Perhaps deep is what we actually want there ;D?

[skestle]

See #144

[MrPowers]

Glad we're experimenting with Scala 2.13 😄

I'm glad we're doing this work to see what bugs need to be fixed before Scala 2.13 gets released. Better to get out ahead of them.

[MrPowers]

@skestle - do we still need this PR?

[skestle]

Nah. It'd be rather complicated supporting 3.0/2.12 and 3.2/2.13...