Add support for custom fixity declarations

[flupe]

Implements #41.

WIP, I have yet to fix infix applications parentheses.

[flupe]

Kinda hitting a roadblock here. Currently pretty-printing infix applications (1 + 2 - 5 instead of 1 + (2 - 5)) is done by processing the generated Haskell syntax and accounting for the default fixities of things in the prelude (this mapping is exported by the Haskell syntax library we use).

My PR works but fixity of custom operators is ignored and each application sees its children parenthesized (which is fine).

In order to also pretty-print custom operators, I wanted the post-processing pass to rely on a mapping from names to fixity of things in scope of the current module. I thought this info could be found in _scopeFixities from ScopeInfo, info we retrieve from iInsideScope of the module Interface that we get when compiling the module. To my surprise, this mapping is always empty, which I find surprising.

Looking for some comments on this, @jespercockx ? The lazy way out is to not pretty-print properly custom operators, which is arguably fine is the point of Agda2HS is to make it easy to bind to generated Haskell, not make said Haskell particularly pretty.

[jespercockx]

Hm that's a bit surprising indeed, I'm not sure why the fixity information is absent. I guess this will require some detective work in the Agda codebase itself to figure out where the fixity information is going to.

I put this PR on the 1.0 milestone because I assumed it was (almost) ready, but since it isn't I propose we keep the current situation as it is.

[flupe]

Just rebased this PR on current master. Just wanted to note that it does work currently (you can introduce your own fixity). The only caveat is that your custom declarations won't be used when pretty-printing the generated Haskell code, but it will generate correct Haskell.

Maybe we can merge and save the problem of pretty-printing custom fixities for later? @jespercockx

[jespercockx]

Sounds good, thanks for the PR!