Support let blocks in nickel

[jneem]

Support let blocks in nickel

Description

Nickel's master branch now has support for "let blocks", where multiple comma-separated bindings can be placed between the "let" and the "in", like

let
a = 1,
b = 2,
in
a + b

This PR introduces support for formatting let blocks, which it handles differently from single let bindings in two ways: the first binding is allowed to go on its own line, and new lines are allowed after each comma.

It would also be possible to indent the bindings, like

let
  a = 1,
  b = 2,
in
a + b

but it feels a bit strange to me to indent the bindings and not the term after "in".

Checklist

Checklist before merging:

• CHANGELOG.md updated
• README.md up-to-date

[yannham]

I vote for the second solution, which is for example what Nix formatters do (and Dhall, I think) and also is more in-line with the way we currently format multi-line let-bindings.

The rationale is that bindings are "one level deeper", but the body is the continuation of the program, and can be huge. And you often chain let-bindings a lot, so you don't want to end up with something like:

let
  a = 1,
  b = 2,
in
  let
    c = a,
    d = b,
  in
    let ...