Add real error messages for associativity errors

[garyb]

Resolves #3310

The code for generating the errors is kinda absurd as it's more complicated than the actual rebracketing code (since using parsec hides a bunch of that), but whatyagonnado.

[garyb]

This case was a little surprising, but GHC has an error for it too:

Prelude> let f x y = f <$> x == f <$> y

<interactive>:10:13:
    Precedence parsing error
        cannot mix ‘<$>’ [infixl 4] and ‘==’ [infix 4] in the same infix expression

<interactive>:10:13:
    Precedence parsing error
        cannot mix ‘==’ [infix 4] and ‘<$>’ [infixl 4] in the same infix expression

[garyb]

GHC again:

Prelude> True == True == True

<interactive>:11:1:
    Precedence parsing error
        cannot mix ‘==’ [infix 4] and ‘==’ [infix 4] in the same infix expression

[hdgarrood]

I think we’ll need to update the docs for fixities now - it sounds to me like the docs I wrote might not all still apply, and there might be cases where the examples in the docs now produce these errors.

[garyb]

Ah ok, I'll open an issue over there as a reminder.