Support let bindings

[janmasrovira]

[mariari]

It would be good to have let here as the special form that has proper sequencing.

Two proposed syntax:

-- option 1 
foo :=
  let bar := 3;
      baz := 10;
  in bar + baz;


-- option 2
foo :=
  let { 
    bar := 3;
    baz := 10;
  } 
  bar + baz;

What should also work is

bar foo :=
  let baz := foo + 10;
      foo := foo + baz;
  in foo + foo

where baz and foo refers to the passed foo. with foo also referring to the previously bound baz.

One thing one should also consider is let-rec blocks versus let blocks. In ocaml, you have let rec along with andto express recursion and dependencies respectively

In Common Lisp one has let vs let* for similar reasons. Having such distinctions allows for better idioms to form.

From here where anoma/juvix#1354 can be implemented as sugar ontop of let. Namely we can do ocaml style and between each value and shove it as a block on the top of the current compilation block.

I would however suggest that block syntax be fleshed out, Since the syntax feels very c inspired, {} may be the most natural choice for blocks but there are issues to over come even there.