added a pure functional implementation of operator precedence parsing

[peteraldous]

Submitted for discussion; needs tests and documentation.

[peteraldous]

I don't particularly care for the fact that the Associativity enum lives outside of the Parsers trait, but I don't see a way to include it because of the binary compatibility constraints. It might be there's a simple solution and I'd be happy to find out about it.

[peteraldous]

I'm still not seeing the same results when I run sbt test locally as the CI pipeline is showing. I've rebased on the new main (as this code is completely orthogonal to the changes there) and addressed the error I saw in the tests. I'll push again if I see more errors. Sorry about the commit spam; I don't know how to reproduce the same errors locally.