Allow omitting 'in' in chained bindings

[vapourismo]

Hi,

I want to make the language change as described below.
I am not sure whether I should propose this with dhall-lang or here.
Any help getting this through the proper channels is greatly appreciated.

Apologies if this has been brought up before, the GitHub search makes it awefully hard to search for let and in.

---

In my head this is a purely cosmetic change.
It should not change the validity of programs that are currently valid.

What was previously written as

   let l = λ(n : Natural) → λ(m : Natural) → λ(x : Natural) → n + m * x
in let f = l 2 3
in f 445

can now be written like

let l = λ(n : Natural) → λ(m : Natural) → λ(x : Natural) → n + m * x
let f = l 2 3
in
    f 445

This effectively makes in terminate a let-chain.

[Gabriella439]

@vapourismo: Changes to the language first go through a standardization process in this repository:

https://github.com/dhall-lang/dhall-lang/

... mainly because we're working on implementations in other languages.

The issue where this was most recently discussed is this one

dhall-lang/dhall-lang#226

The summary of that issue is I avoid introducing syntactic sugar unless it leads to significant code compression.

There was an additional point made in that issue about using records as a replacement, but that would not be relevant to your example since the definition of f depends on the definition of l so a record would not serve as a substitute.

[vapourismo]

The removal of most ins and the redundant whitespace counts as significant code compression, especially in a language where let is the preferred way to bind things.
Moreover it cleans things up a little.

[Gabriella439]

I'll close this in favor of #675, which supersedes this change