Inconsistent naming for arguments

[jfmengels]

For functions (in declarations and let declarations), the arguments are under a field named "arguments".
For lambdas, the arguments are under the name "args".

I think we should rename "args" to "arguments" for lambdas.

[lenards]

Jeroen, I realize that this is something someone might already have waiting to PR - but I did a quick rename on this:

• master...lenards:elm-syntax:master

I did run elm make --docs=docs.json, but I'm not still seeing "args" within the docs.json, so I might not have that handled correctly.

If the above diff looks like what you expected, I can put it up as a pull request.

[lenards]

(I've been trying to use elm-syntax to help with an i18n extractor; thought I'd try to help out - even if it's really small thing)

🙇