swift234

Implement SE-0077: precedence group declarations.

What I've implemented here deviates from the current proposal text
in the following ways:

- I had to introduce a FunctionArrowPrecedence to capture the parsing
  of -> in expression contexts.

- I found it convenient to continue to model the assignment property
  explicitly.

- The comparison and casting operators have historically been
  non-associative; I have chosen to preserve that, since I don't
  think this proposal intended to change it.

- This uses the precedence group names and higherThan/lowerThan
  as agreed in discussion.