let function syntax

[francois-caddet]

Extend the language to accept ML like functions declaration:

let f a b = a + b in f 3 5

discuced in #207
Can accept patterns params like in #81 but with this new syntax. Only the first element after the let has to be an identifier, because it's the name of the function but the others can be patterns.
@yannham is there an other issue to link?

[yannham]

I think we also want destructuring to work on such function definitions.

[yannham]

Oh, and we also want those when defining record fields, like { foo : Num -> Num, foo x y = x + y}

[mboes]

I think we want to punt on this to post release and until we have more code written in Nickel, for the same reason as discussed in #494 (comment).

[yannham]

Even in the light of #494 (comment) ?

[francois-caddet]

I don't know well this syntax but will see. If it's not a lot to implement I will do it for this PR.

[mboes]

Sorry, what I meant in that comment is that this clearly needs a solution eventually. But since this is only syntactic sugar, would be good to gain experience first. If there was only one possible design here that would be one thing. But multiple designs are reasonable, including changing the syntax for functions everywhere, so that no syntactic sugar for lets is necessary. Further, it feels to me like there are higher priorities ahead of the release.

[francois-caddet]

So we'll see later.