nickel: Block syntax for let-binding

Is your feature request related to a problem? Please describe. Block let-binding are nice to express a series of order-independent bindings. See #218 for more context.

Describe the solution you’d like As decided during the standardization meeting, have let-binding blocks as in Nix, just replacing semicolon ; by , for consistency with the rest of the Nickel syntax.

Example:

let foo = ...,
    bar = ...,
    blah = ...,
in
...

To be consistent, the inner syntax should be exactly the same as for records, just without the braces. Those let-binding would also be mutually recursive, as records.

Describe alternatives you’ve considered See #218 for discussion and alternatives.