Implement no associative infix op for precedence!{}

[A4-Tacks]

Like a:@ "<" b:@ {}

Part of #404

[kevinmehall]

Rebased this and tweaked the test to use "<" and ">" as the example non-associative operators rather than "+" to demonstrate an example use case.

But I found a problem with it: I added a left-associative "||" that binds weaker than "<" and ">" but this doesn't work correctly -- it should be allowed to have an associative operator with non-associative arguments, but the new boolean result propagates up and prevents that operator from being parsed.

[A4-Tacks]

I have implemented a tests passed parser

But I suspect it has more hidden problems, and it's a bit unsightly

(The println!() used for debugging has not been removed)