[ fix #273 ] Add more parenthesis in arguments of operators and sections

[jespercockx]

Ideally, our pretty-printing library should insert parenthesis where needed. However, hs-src-exts does not insert adequate parenthesis for infix operators so we need to insert some by hand (see issues #54 and #273).

This also disables the printing of _- u as a right section (- u) in Haskell, as that would be interpreted as a negation instead.

[omelkonian]

Introducing \ section -> section - ... is quite ugly; why not use a left section on subtract = flip (-) instead?

            | a == hsVar x
            -> if pp op == "-" then -- no right section for minus, as Haskell parses this as negation!
                   Hs.LeftSection () (QConOp $ unQual "subtract") b 
                else
                   Hs.RightSection () op b
             -- System-inserted visible lambdas can only come from sections

Minor comments:

1. No need for the intermediate variable hsx, just inline here
2. No need to mention the name of the person writing the comment :P

[jespercockx]

For this PR I wasn't really looking at whether the output is pretty, just whether it is correct. But I agree that your version is prettier.

The variable hsx was already there, I didn't want to touch it since it's unrelated to this PR.

Writing my name in comments is a habit I got from working on Agda itself where it seems to be standard practice. I personally quite like these little bits of personality in the codebase :)